BUG?: 1.2.5 `astype('bool')` converts NaNs in `'category'` columns to `True`
- [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.

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

---

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

```python
pd.Series(['a','b',None], dtype='category').astype('bool')
```

In 1.2.5 produces:
```python
0    True
1    True
2    True
dtype: bool
```

In 1.2.4 produces: 
```python
0    True
1    True
2     NaN
dtype: object
```

#### Problem description
At first, I thought this was a bug, but the `astype` call producing `object` dtype only started showing up in 1.2.0, so I went back and checked 1.1.5, and 1.2.5 now matches what was on 1.1.5.

This makes me think that the behavior in 1.1.5 and 1.2.5 is the expected behavior, but I don't see a release note in 1.1.5 referencing this change back to the expected, so I just wanted to point this out incase it was an unknown regression/fix or a bug of some sort.

I do see that #41797 seems like a similar problem that also first showed up in 1.2.0 and was fixed in 1.2.5, so I'm wondering if they're related.


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

<details>

INSTALLED VERSIONS
------------------
commit           : 7c48ff4409c622c582c56a5702373f726de08e96
python           : 3.8.2.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.6.0
Version          : Darwin Kernel Version 19.6.0: Sun Jul  5 00:43:10 PDT 2020; root:xnu-6153.141.1~9/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.5
numpy            : 1.20.3
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 21.1.2
setuptools       : 41.2.0
Cython           : None
pytest           : 6.0.1
hypothesis       : None
sphinx           : 3.2.1
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.3
IPython          : 7.18.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : 2021.06.0
fastparquet      : 0.5.0
gcsfs            : None
matplotlib       : 3.2.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 4.0.1
pyxlsb           : None
s3fs             : None
scipy            : 1.6.3
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : 0.53.1

</details>
