BUG: .value_counts converts pd.NA to np.nan
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- [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

```python
pd.Series(["a", "a", "b", pd.NA]).value_counts(dropna=False).index
Index(['a', 'b', nan], dtype='object')
```

#### Problem description

`.value_counts` converts `pd.NA` to `np.nan`. Instead `pd.NA` should be preserved. In my use case, I use `value_counts` to find small groups, based on which I want to filter the data. This is hard, if the value in the counts differs from the actual value.
 
#### Expected Output
```python
Index(['a', 'b', <NA>], dtype='object')
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : c7f7443c1bad8262358114d5e88cd9c8a308e8aa
python           : 3.7.9.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.4.0-80-generic
Version          : #90-Ubuntu SMP Fri Jul 9 22:49:44 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            : 1.20.2
pytz             : 2020.5
dateutil         : 2.8.1
pip              : 19.3.1
setuptools       : 49.6.0.post20210108
Cython           : None
pytest           : 6.1.2
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.16.1
pandas_datareader: None
bs4              : None
bottleneck       : 1.3.2
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.2.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 3.0.0
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.13
tables           : None
tabulate         : 0.8.7
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
