BUG: fillna(0) does not fill NaNs for Float64Arrays
- [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.

---

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

```python
f = pandas.DataFrame(data = [[0, 0]], columns = ['A', 'B']).astype('Int64')
f['C'] = f['A'] / f['B']
f.fillna(0)
```

Output:
```
	A	B	C
0	0	0	NaN
```

#### Problem description

fillna does not work as expected. It do not fill na

#### Expected Output
```
	A	B	C
0	0	0	0
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : 7d32926db8f7541c356066dcadabf854487738de
python           : 3.7.9.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.18362
machine          : AMD64
processor        : Intel64 Family 6 Model 142 Stepping 10, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 1.2.2
numpy            : 1.18.3
pytz             : 2020.5
dateutil         : 2.8.1
pip              : 20.3.3
setuptools       : 51.3.3.post20210118
Cython           : 0.29.21
pytest           : 6.2.1
hypothesis       : None
sphinx           : 3.4.3
blosc            : None
feather          : None
xlsxwriter       : 1.3.7
lxml.etree       : 4.6.2
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.19.0
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : 1.3.2
fsspec           : 0.8.3
fastparquet      : 0.3.3
gcsfs            : None
matplotlib       : 3.2.2
numexpr          : 2.7.2
odfpy            : None
openpyxl         : 3.0.6
pandas_gbq       : None
pyarrow          : 0.16.0
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : 1.3.21
tables           : 3.6.1
tabulate         : 0.8.7
xarray           : None
xlrd             : 2.0.1
xlwt             : 1.3.0
numba            : 0.51.2

</details>
