BUG: "ValueError: Must provide strings." when using "string" as dtype
- [x] I have checked that this issue has not already been reported.

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

- [ ] (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

Given a csv file `test.csv`(sorry, didn't manage to reproduce without reading a `csv`) with the following content:
```
x,y
00000114136524405793555657657729584885,
```

```python
df = pd.read_csv('test.csv', dtype={'x': 'string', 'y': 'string'})
df['y'].fillna({}, inplace=True)
```

#### Problem description

Above code fails with an error `ValueError: Must provide strings.`. Full traceback below in the details.

<details>

```
Traceback (most recent call last):
  File "test.py", line 3, in <module>
    df['y'].fillna({}, inplace=True)
  File "/home/user/.local/lib/python3.8/site-packages/pandas/core/series.py", line 4529, in fillna
    return super().fillna(
  File "/home/user/.local/lib/python3.8/site-packages/pandas/core/generic.py", line 6057, in fillna
    new_data = self._mgr.fillna(
  File "/home/user/.local/lib/python3.8/site-packages/pandas/core/internals/managers.py", line 585, in fillna
    return self.apply(
  File "/home/user/.local/lib/python3.8/site-packages/pandas/core/internals/managers.py", line 406, in apply
    applied = getattr(b, f)(**kwargs)
  File "/home/user/.local/lib/python3.8/site-packages/pandas/core/internals/blocks.py", line 1779, in fillna
    values = values.fillna(value=value, limit=limit)
  File "/home/user/.local/lib/python3.8/site-packages/pandas/core/arrays/string_.py", line 262, in fillna
    return super().fillna(value, method, limit)
  File "/home/user/.local/lib/python3.8/site-packages/pandas/core/arrays/numpy_.py", line 307, in fillna
    new_values[mask] = value
  File "/home/user/.local/lib/python3.8/site-packages/pandas/core/arrays/string_.py", line 256, in __setitem__
    raise ValueError("Must provide strings.")
ValueError: Must provide strings.
```

</details>

On the other hand, when the `dtype` object is `{'x': str, 'y': str}`, the error is not raised - from [the docs](https://pandas.pydata.org/pandas-docs/stable/user_guide/basics.html#dtypes) I understood both versions should be equivalent?

#### Expected Output
Both versions have the same behavior.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : c7f7443c1bad8262358114d5e88cd9c8a308e8aa
python           : 3.8.10.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.11.0-25-generic
Version          : #27~20.04.1-Ubuntu SMP Tue Jul 13 17:41:23 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.3.1
numpy            : 1.21.1
pytz             : 2021.1
dateutil         : 2.8.2
pip              : 21.2.2
setuptools       : 44.0.0
Cython           : None
pytest           : 6.2.4
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : 2021.07.0
fastparquet      : None
gcsfs            : 2021.07.0
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None
None

</details>
