TST: Empty Series.reindex with tz-aware dtype
- [x] closes #20869
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @mroeschke! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/test_multilevel.py`](https://github.com/mroeschke/pandas/blob/ab05b877832c4da2b29f71757ec8c18c6baa856c/pandas/tests/test_multilevel.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23480?src=pr&el=h1) Report
> Merging [#23480](https://codecov.io/gh/pandas-dev/pandas/pull/23480?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8586644429a8c0054a74e645830601419da6613f?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23480/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23480?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23480   +/-   ##
=======================================
  Coverage   92.23%   92.23%           
=======================================
  Files         161      161           
  Lines       51197    51197           
=======================================
  Hits        47220    47220           
  Misses       3977     3977
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <ø> (ø)` | :arrow_up: |
| #single | `42.27% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23480?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23480/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `96.41% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/23480/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9hcGkucHk=) | `99% <0%> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/23480/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.81% <0%> (ø)` | :arrow_up: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23480/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `98.84% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23480?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/23480?src=pr&el=footer). Last update [8586644...ac5cd2d](https://codecov.io/gh/pandas-dev/pandas/pull/23480?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks