BUG: fix HDFStore.append with all empty strings error (GH12242)
- [x] closes #12242
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Hello @josham! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/io/pytables.py`](https://github.com/josham/pandas/blob/e9fd189b939699ee69141e089ac20f86d39c4afb/pandas/io/pytables.py) !

 - There are no PEP8 issues in the file [`pandas/tests/io/test_pytables.py`](https://github.com/josham/pandas/blob/e9fd189b939699ee69141e089ac20f86d39c4afb/pandas/tests/io/test_pytables.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23435?src=pr&el=h1) Report
> Merging [#23435](https://codecov.io/gh/pandas-dev/pandas/pull/23435?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a4f9a44e8e348744e2dc95f243c44e9829546d5f?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23435/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23435?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23435      +/-   ##
==========================================
- Coverage   92.21%   92.21%   -0.01%     
==========================================
  Files         161      161              
  Lines       51189    51187       -2     
==========================================
- Hits        47204    47202       -2     
  Misses       3985     3985
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.64% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.22% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23435?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/23435/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `92.43% <100%> (ø)` | :arrow_up: |
| [pandas/core/reshape/melt.py](https://codecov.io/gh/pandas-dev/pandas/pull/23435/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZWx0LnB5) | `97.32% <0%> (-0.03%)` | :arrow_down: |
| [pandas/core/reshape/reshape.py](https://codecov.io/gh/pandas-dev/pandas/pull/23435/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9yZXNoYXBlLnB5) | `99.54% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/23435/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `96.55% <0%> (ø)` | :arrow_up: |
| [pandas/core/reshape/tile.py](https://codecov.io/gh/pandas-dev/pandas/pull/23435/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS90aWxlLnB5) | `94.73% <0%> (ø)` | :arrow_up: |
| [pandas/core/reshape/merge.py](https://codecov.io/gh/pandas-dev/pandas/pull/23435/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZXJnZS5weQ==) | `94.01% <0%> (ø)` | :arrow_up: |
| [pandas/core/reshape/util.py](https://codecov.io/gh/pandas-dev/pandas/pull/23435/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS91dGlsLnB5) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/core/reshape/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/23435/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9jb25jYXQucHk=) | `97.6% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23435?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/23435?src=pr&el=footer). Last update [a4f9a44...d27fe02](https://codecov.io/gh/pandas-dev/pandas/pull/23435?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 can you rebase again, the whatsnew is conflicted. ping on green. lgtm. ping on green. @jreback green thanks @josham 