BUG: Dataframe mask method does not work properly with pd.StringDtype()
- [x] I have checked that this issue has not already been reported.

- [ ] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
import pandas as pd

df = pd.DataFrame(
    {
        "A": ["foo", "bar", "baz", pd.NA]
    }, index=["id1", "id2", "id3", "id4"], dtype=pd.StringDtype()
)

filtered_df = pd.DataFrame(
    {
        "A": ["this", "that"]
    }, index=["id2", "id3"], dtype=pd.StringDtype()
)

filter = pd.Series([False, True, True, False])

new_df = df.mask(filter, filtered_df)
```

#### Problem description

Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "C:\Program Files\JetBrains\PyCharm 2020.3.2\plugins\python\helpers\pydev\_pydev_bundle\pydev_umd.py", line 197, in runfile
    pydev_imports.execfile(filename, global_vars, local_vars)  # execute the script
  File "C:\Program Files\JetBrains\PyCharm 2020.3.2\plugins\python\helpers\pydev\_pydev_imps\_pydev_execfile.py", line 18, in execfile
    exec(compile(contents+"\n", file, 'exec'), glob, loc)
  File "C:/Users/XXX/AppData/Roaming/JetBrains/PyCharm2020.3/scratches/scratch_24.py", line 19, in <module>
    new_df = df.mask(filter, filtered_df)
  File "C:\Users\XXX\AppData\Local\pypoetry\Cache\virtualenvs\basel-w19Rd76b-py3.9\lib\site-packages\pandas\core\generic.py", line 9317, in mask
    return self.where(
  File "C:\Users\XXX\AppData\Local\pypoetry\Cache\virtualenvs\basel-w19Rd76b-py3.9\lib\site-packages\pandas\core\generic.py", line 9286, in where
    return self._where(
  File "C:\Users\XXX\AppData\Local\pypoetry\Cache\virtualenvs\basel-w19Rd76b-py3.9\lib\site-packages\pandas\core\generic.py", line 9056, in _where
    _, other = self.align(
  File "C:\Users\XXX\AppData\Local\pypoetry\Cache\virtualenvs\basel-w19Rd76b-py3.9\lib\site-packages\pandas\core\frame.py", line 4102, in align
    return super().align(
  File "C:\Users\XXX\AppData\Local\pypoetry\Cache\virtualenvs\basel-w19Rd76b-py3.9\lib\site-packages\pandas\core\generic.py", line 8813, in align
    return self._align_frame(
  File "C:\Users\XXX\AppData\Local\pypoetry\Cache\virtualenvs\basel-w19Rd76b-py3.9\lib\site-packages\pandas\core\generic.py", line 8880, in _align_frame
    right = other._reindex_with_indexers(
  File "C:\Users\XXX\AppData\Local\pypoetry\Cache\virtualenvs\basel-w19Rd76b-py3.9\lib\site-packages\pandas\core\generic.py", line 4877, in _reindex_with_indexers
    new_data = new_data.reindex_indexer(
  File "C:\Users\XXX\AppData\Local\pypoetry\Cache\virtualenvs\basel-w19Rd76b-py3.9\lib\site-packages\pandas\core\internals\managers.py", line 1311, in reindex_indexer
    new_blocks = [
  File "C:\Users\XXX\AppData\Local\pypoetry\Cache\virtualenvs\basel-w19Rd76b-py3.9\lib\site-packages\pandas\core\internals\managers.py", line 1312, in <listcomp>
    blk.take_nd(
  File "C:\Users\XXX\AppData\Local\pypoetry\Cache\virtualenvs\basel-w19Rd76b-py3.9\lib\site-packages\pandas\core\internals\blocks.py", line 1863, in take_nd
    new_values = self.values.take(indexer, fill_value=fill_value, allow_fill=True)
  File "C:\Users\XXX\AppData\Local\pypoetry\Cache\virtualenvs\basel-w19Rd76b-py3.9\lib\site-packages\pandas\core\arrays\_mixins.py", line 78, in take
    return self._from_backing_data(new_data)
  File "C:\Users\XXX\AppData\Local\pypoetry\Cache\virtualenvs\basel-w19Rd76b-py3.9\lib\site-packages\pandas\core\arrays\numpy_.py", line 190, in _from_backing_data
    return type(self)(arr)
  File "C:\Users\XXX\AppData\Local\pypoetry\Cache\virtualenvs\basel-w19Rd76b-py3.9\lib\site-packages\pandas\core\arrays\string_.py", line 195, in __init__
    self._validate()
  File "C:\Users\XXX\AppData\Local\pypoetry\Cache\virtualenvs\basel-w19Rd76b-py3.9\lib\site-packages\pandas\core\arrays\string_.py", line 200, in _validate
    raise ValueError("StringArray requires a sequence of strings or pandas.NA")
ValueError: StringArray requires a sequence of strings or pandas.NA

In generic.py starting on line 9053, there is this code block:
```python
            # align with me
            if other.ndim <= self.ndim:

                _, other = self.align(
                    other, join="left", axis=axis, level=level, fill_value=np.nan
                )
```
In case that the filtered_df has fewer rows than the original df, this code block is executed and it uses np.nan as a fill_value for the align method. This fill_value will propagate until the error is raised as shown above in the stack trace. I think that this issue can be easily solved if the fill_value is a mutable parameter for the .mask or .where method

#### Expected Output
"""    
----| A
id1 |   \<NA\>
id2 |  this
id3 |  that
id4 |   \<NA\>
"""
or 
"""
----|  A
id1 |  foo
id2 |  this
id3 |  that
id4 |   \<NA\>
"""
#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : f2c8480af2f25efdbd803218b9d87980f416563e
python           : 3.9.1.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.19041
machine          : AMD64
processor        : Intel64 Family 6 Model 79 Stepping 1, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : English_United States.1252
pandas           : 1.2.3
numpy            : 1.20.2
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.0.1
setuptools       : 52.0.0
Cython           : None
pytest           : 6.2.2
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.3
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : 3.0.7
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : 1.6.1
sqlalchemy       : 1.4.3
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
numba            : None

</details>
