BUG: Series.fillna raising with `float32` dtype when using value arg
- [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
ser = pd.Series([1.0, 2.0], dtype="float32")
val = {1: 1.01}
ser.fillna(val)
```

#### Problem description
The above code raises with 
`TypeError: Cannot cast array data from dtype('float64') to dtype('float32') according to the rule 'safe'`

#### Expected Output
```
0    1.0
1    2.0
dtype: float32
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : 343ac2a4179fdd1cb24633e9b95cbec33371c14d
python           : 3.8.10.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 20.4.0
Version          : Darwin Kernel Version 20.4.0: Thu Apr 22 21:46:47 PDT 2021; root:xnu-7195.101.2~1/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.UTF-8

pandas           : 1.4.0.dev0+599.g343ac2a417.dirty
numpy            : 1.21.1
pytz             : 2021.1
dateutil         : 2.8.2
pip              : 21.2.1
setuptools       : 49.6.0.post20210108
Cython           : 0.29.24
pytest           : 6.2.4
hypothesis       : 6.14.3
sphinx           : 3.5.4
blosc            : None
feather          : None
xlsxwriter       : 1.4.4
lxml.etree       : 4.6.3
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 3.0.1
IPython          : 7.25.0
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : 1.3.2
fsspec           : 2021.05.0
fastparquet      : 0.6.3
gcsfs            : 2021.05.0
matplotlib       : 3.4.2
numexpr          : 2.7.3
odfpy            : None
openpyxl         : 3.0.7
pandas_gbq       : None
pyarrow          : 4.0.1
pyxlsb           : None
s3fs             : 0.4.2
scipy            : 1.7.0
sqlalchemy       : 1.4.22
tables           : 3.6.1
tabulate         : 0.8.9
xarray           : 0.19.0
xlrd             : 2.0.1
xlwt             : 1.3.0
numba            : 0.53.1

</details>
