BUG: drop_duplicates on categorical not preserving NA
### 

- [X] I have checked that this issue has not already been reported.

- [X] I have confirmed this bug exists on the [latest version](https://pandas.pydata.org/docs/whatsnew/index.html) of pandas.

- [X] I have confirmed this bug exists on the master branch of pandas.


### Reproducible Example

```python
ser = Series(
    Categorical(
        [True, False, True, False, NA], categories=[True, False], ordered=True
    )
)
ser.drop_duplicates()
```


### Issue Description

``NA`` gets converted to ``NaN``

```
0     True
1    False
4      NaN
dtype: category
Categories (2, object): [True < False]
```


### Expected Behavior

Returning ``NA``

```
0     True
1    False
4       NA
dtype: category
Categories (2, object): [True < False]
```

### Installed Versions

<details>
INSTALLED VERSIONS
------------------
commit           : 01b86edbbbf4bd81d85d7d6638117545cd13dfac
python           : 3.8.12.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.11.0-38-generic
Version          : #42~20.04.1-Ubuntu SMP Tue Sep 28 20:41:07 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.4.0.dev0+1085.g01b86edbbb
numpy            : 1.21.3
pytz             : 2021.1
dateutil         : 2.8.2
pip              : 21.2.4
setuptools       : 58.0.4
Cython           : 0.29.24
pytest           : 6.2.5
hypothesis       : 6.23.1
sphinx           : 4.2.0
blosc            : None
feather          : None
xlsxwriter       : 3.0.1
lxml.etree       : 4.6.3
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 3.0.1
IPython          : 7.28.0
pandas_datareader: None
bs4              : 4.10.0
bottleneck       : 1.3.2
fsspec           : 2021.11.0
fastparquet      : 0.7.1
gcsfs            : 2021.05.0
matplotlib       : 3.4.3
numexpr          : 2.7.3
odfpy            : None
openpyxl         : 3.0.9
pandas_gbq       : None
pyarrow          : 5.0.0
pyxlsb           : None
s3fs             : 2021.11.0
scipy            : 1.7.2
sqlalchemy       : 1.4.25
tables           : 3.6.1
tabulate         : 0.8.9
xarray           : 0.18.0
xlrd             : 2.0.1
xlwt             : 1.3.0
numba            : 0.53.1
None

Process finished with exit code 0


</details>
