BUG: Cannot assign Series value as None on initial creation of the item

[petebachant]

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 numpy as np
import pandas as pd

s = pd.Series(dtype=object)
s["none"] = None
s["nan"] = np.nan

assert s["none"] is None  # Fails

s["none"] = None

assert s["none"] is None  # Passes

Issue Description

Upon initial creation of an item in a Series, None is coerced to NaN, even if dtype=object. After the item exists, it is then possible to set as None.

Expected Behavior

Expect the example above to not raise an exception.

Installed Versions

INSTALLED VERSIONS ------------------ commit : 87cfe4e python : 3.9.13.final.0 python-bits : 64 OS : Linux OS-release : 5.15.68.1-microsoft-standard-WSL2 Version : #1 SMP Mon Sep 19 19:14:52 UTC 2022 machine : x86_64 processor : x86_64 byteorder : little LC_ALL : None LANG : C.UTF-8 LOCALE : en_US.UTF-8

pandas : 1.5.0
numpy : 1.23.3
pytz : 2022.1
dateutil : 2.8.2
setuptools : 65.4.1
pip : 22.2.2
Cython : None
pytest : 7.1.3
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : 3.0.3
lxml.etree : 4.9.1
html5lib : None
pymysql : 1.0.2
psycopg2 : None
jinja2 : 3.1.2
IPython : 8.5.0
pandas_datareader: None
bs4 : 4.11.1
bottleneck : None
brotli :
fastparquet : 0.8.3
fsspec : 2022.8.2
gcsfs : None
matplotlib : 3.6.0
numba : None
numexpr : 2.8.3
odfpy : None
openpyxl : 3.0.10
pandas_gbq : None
pyarrow : 6.0.1
pyreadstat : None
pyxlsb : None
s3fs : 0.4.2
scipy : 1.8.1
snappy : None
sqlalchemy : 1.4.41
tables : 3.7.0
tabulate : 0.9.0
xarray : 2022.9.0
xlrd : 2.0.1
xlwt : None
zstandard : None
tzdata : None

[phofl]

Hi, thanks for your report. Duplicate of #48665