BUG: None value converted to NaN in Series

[stephanecollot]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

>>> import pandas as pd
>>> s = pd.Series(['a', 'b'])
>>> s
0    a
1    b
dtype: object
>>> s['a'] = None
>>> s
0      a
1      b
a    NaN
dtype: object

Issue Description

In 1.5.0 there is a behaviour change, if you set a value on a Serie to None it is replaced with NaN.

Same thing with s.loc['dd'] = None

Expected Behavior

It should stay None

Installed Versions

>>> pd.show_versions()

INSTALLED VERSIONS

commit : 9a607e2
python : 3.8.13.final.0
python-bits : 64
OS : Darwin
OS-release : 21.6.0
Version : Darwin Kernel Version 21.6.0: Wed Aug 10 14:25:27 PDT 2022; root:xnu-8020.141.5~2/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : en_GB.UTF-8
LOCALE : en_GB.UTF-8

pandas : 1.6.0.dev0+165.g9a607e2afa
numpy : 1.22.3
pytz : 2021.3
dateutil : 2.8.2
setuptools : 56.0.0
pip : 22.0.4
Cython : 0.29.32
pytest : 6.2.5
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : 1.1
pymysql : None
psycopg2 : None
jinja2 : 3.0.3
IPython : 7.16.1
pandas_datareader: None
bs4 : 4.11.1
bottleneck : None
brotli : None
fastparquet : None
fsspec : 0.8.7
gcsfs : None
matplotlib : 3.5.1
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : 3.0.0
pyreadstat : None
pyxlsb : None
s3fs : 0.3.5
scipy : 1.5.4
snappy : None
sqlalchemy : 1.4.32
tables : None
tabulate : 0.8.9
xarray : None
xlrd : None
xlwt : None
zstandard : None
tzdata : None

[phofl]

Can confirm (as I expected :))

commit bd9a6f09264c58ef402d4c7faf5fc9d48c159194
Author: Patrick Hoefler <[email protected]>
Date:   Sat Jul 2 00:18:05 2022 +0200

    BUG: Series.setitem losing precision when enlarging (#47342)

I'll look into this