BUG: Pie chart with categorical values fails
- [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.

---

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

```python
import pandas as pd
pd.Series(pd.Categorical(['cat'], categories=['cat'], ordered=True)).plot.pie()
```

#### Problem description

It should be possible to plot a pie chart with categorical values.

Traceback:
<details><pre>
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3/dist-packages/pandas/plotting/_core.py", line 1528, in pie
    return self(kind="pie", **kwargs)
  File "/usr/lib/python3/dist-packages/pandas/plotting/_core.py", line 949, in __call__
    return plot_backend.plot(data, kind=kind, **kwargs)
  File "/usr/lib/python3/dist-packages/pandas/plotting/_matplotlib/__init__.py", line 60, in plot
    plot_obj = PLOT_CLASSES[kind](data, **kwargs)
  File "/usr/lib/python3/dist-packages/pandas/plotting/_matplotlib/core.py", line 1506, in __init__
    data = data.fillna(value=0)
  File "/usr/lib/python3/dist-packages/pandas/core/series.py", line 4530, in fillna
    return super().fillna(
  File "/usr/lib/python3/dist-packages/pandas/core/generic.py", line 6061, in fillna
    new_data = self._mgr.fillna(
  File "/usr/lib/python3/dist-packages/pandas/core/internals/managers.py", line 594, in fillna
    return self.apply(
  File "/usr/lib/python3/dist-packages/pandas/core/internals/managers.py", line 409, in apply
    applied = getattr(b, f)(**kwargs)
  File "/usr/lib/python3/dist-packages/pandas/core/internals/blocks.py", line 1779, in fillna
    values = values.fillna(value=value, limit=limit)
  File "/usr/lib/python3/dist-packages/pandas/core/arrays/categorical.py", line 1721, in fillna
    raise ValueError("fill value must be in categories")
ValueError: fill value must be in categories
</pre></details>

#### Expected Output

It should generate pie chart without errors.

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

<details>
INSTALLED VERSIONS
------------------
commit           : b5958ee1999e9aead1938c0bba2b674378807b3d
python           : 3.9.5.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.8.0-50-generic
Version          : #56-Ubuntu SMP Mon Apr 12 17:18:36 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.1.5
numpy            : 1.19.5
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 20.3.4
setuptools       : 52.0.0
Cython           : 0.29.21
pytest           : 6.0.2
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.6.3
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : None
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : 1.2.1
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.4
numexpr          : 2.7.2
odfpy            : None
openpyxl         : 3.0.3
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.6.0
sqlalchemy       : None
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : 1.3.0
numba            : None
</details>

####  Additional context

Might be linked to #39320 and #32414

