BUG: Handle all-NA blocks in concat
Previously we special cased all-na blocks. We should only do that for
non-extension dtypes.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20382?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@13ae107`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20382/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20382?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #20382   +/-   ##
=========================================
  Coverage          ?   91.79%           
=========================================
  Files             ?      152           
  Lines             ?    49184           
  Branches          ?        0           
=========================================
  Hits              ?    45148           
  Misses            ?     4036           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.17% <ø> (?)` | |
| #single | `41.83% <ø> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20382?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/20382/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `95.54% <ø> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20382?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/20382?src=pr&el=footer). Last update [13ae107...c12a24d](https://codecov.io/gh/pandas-dev/pandas/pull/20382?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. The linting error was fixed on master: `pandas/_libs/tslibs/period.pyx:1358:12: W291 trailing whitespace`