BUG: Don't segfault to_numeric when input is empty
Title is self-explanatory.

Closes #16302.
lgtm. ping on green. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16305?src=pr&el=h1) Report
> Merging [#16305](https://codecov.io/gh/pandas-dev/pandas/pull/16305?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ce4eef3750052cec62ca0fe6536521dec523cd64?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16305/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16305?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16305      +/-   ##
==========================================
- Coverage   90.39%   90.37%   -0.03%     
==========================================
  Files         161      161              
  Lines       50863    50863              
==========================================
- Hits        45977    45966      -11     
- Misses       4886     4897      +11
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.15% <ø> (-0.01%)` | :arrow_down: |
| #single | `40.33% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16305?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16305?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/16305?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `80.21% <0%> (-0.2%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16305?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.59% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16305?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16305?src=pr&el=footer). Last update [ce4eef3...9d79605](https://codecov.io/gh/pandas-dev/pandas/pull/16305?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback : Everything is green and ready to go. Thanks!