BUG: resample and apply modify the index type for empty Series
 - [x] closes #14313
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master -u -- "*.py" | flake8 --diff``
 - [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=h1) Report
> Merging [#17149](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/611d29606263d12ecdcc38a2b8a790e99aa443d6?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17149/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17149      +/-   ##
==========================================
- Coverage   91.05%   91.03%   -0.02%     
==========================================
  Files         161      161              
  Lines       49417    49421       +4     
==========================================
- Hits        44995    44990       -5     
- Misses       4422     4431       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.8% <100%> (ø)` | :arrow_up: |
| #single | `40.28% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `96.16% <100%> (+0.02%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.66% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=footer). Last update [611d296...d13c281](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=h1) Report
> Merging [#17149](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/611d29606263d12ecdcc38a2b8a790e99aa443d6?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17149/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17149      +/-   ##
==========================================
- Coverage   91.05%   91.03%   -0.02%     
==========================================
  Files         161      161              
  Lines       49417    49421       +4     
==========================================
- Hits        44995    44990       -5     
- Misses       4422     4431       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.8% <100%> (ø)` | :arrow_up: |
| #single | `40.28% <0%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `96.16% <100%> (+0.02%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.66% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=footer). Last update [611d296...d13c281](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=h1) Report
> Merging [#17149](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8e6b09ff3a09de58e82da6dcabbfddba61a743d6?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17149/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17149      +/-   ##
==========================================
- Coverage   91.01%      91%   -0.02%     
==========================================
  Files         162      162              
  Lines       49497    49501       +4     
==========================================
- Hits        45051    45046       -5     
- Misses       4446     4455       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.77% <100%> (ø)` | :arrow_up: |
| #single | `40.25% <20%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `96.16% <100%> (+0.02%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.66% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=footer). Last update [8e6b09f...3088bcc](https://codecov.io/gh/pandas-dev/pandas/pull/17149?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback  thanks!