REGR: NA-values in ctors with string dtype
```python
In [1]: import pandas as pd
In [2]: pd.Series([1, 2, None], dtype='str')[2]  # None

```

Closes #21083
Did you also add a test case for https://github.com/pandas-dev/pandas/issues/21270 ? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21366?src=pr&el=h1) Report
> Merging [#21366](https://codecov.io/gh/pandas-dev/pandas/pull/21366?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7f6ea676808a39801e1d36ec1db48f269fa4e5a9?src=pr&el=desc) will **decrease** coverage by `0.03%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21366/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21366?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21366      +/-   ##
==========================================
- Coverage   91.89%   91.85%   -0.04%     
==========================================
  Files         153      153              
  Lines       49579    49569      -10     
==========================================
- Hits        45559    45532      -27     
- Misses       4020     4037      +17
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.25% <100%> (-0.04%)` | :arrow_down: |
| #single | `41.87% <100%> (+0.01%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21366?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/21366/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.15% <100%> (-0.03%)` | :arrow_down: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/21366/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `82.39% <0%> (-1.15%)` | :arrow_down: |
| [pandas/core/dtypes/missing.py](https://codecov.io/gh/pandas-dev/pandas/pull/21366/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL21pc3NpbmcucHk=) | `91.95% <0%> (-0.58%)` | :arrow_down: |
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/21366/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `96.97% <0%> (-0.06%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21366?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/21366?src=pr&el=footer). Last update [7f6ea67...4de6f7a](https://codecov.io/gh/pandas-dev/pandas/pull/21366?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @TomAugspurger 

we should prob create an issue about refactor / clean dtypes.cast