BUG: pd.read_csv(io.StringIO("a\nNone")).a[0] is 'None' on pandas 1 but NaN on pandas 2

[graingert]

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
import io

pd.read_csv(io.StringIO("a\nNone")).a[0]

Issue Description

BUG: pd.read_csv(io.StringIO("a\nNone")).a[0] is 'None' on pandas 1 but NaN on pandas 2

Expected Behavior

should be "None"

Installed Versions

INSTALLED VERSIONS

commit : 478d340
python : 3.11.0.final.0
python-bits : 64
OS : Linux
OS-release : 5.19.0-38-generic
Version : #39~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Fri Mar 17 21:16:15 UTC 2
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_GB.UTF-8
LOCALE : en_GB.UTF-8

pandas : 2.0.0
numpy : 1.24.2
pytz : 2022.7.1
dateutil : 2.8.2
setuptools : 66.1.1
pip : 23.0.1
Cython : None
pytest : 7.2.2
hypothesis : None
sphinx : 5.3.0
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.9.2
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.1.2
IPython : 8.12.0
pandas_datareader: 0.10.0
bs4 : 4.12.1
bottleneck : None
brotli :
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : 3.6.3
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : 1.10.1
snappy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
zstandard : None
tzdata : 2023.3
qtpy : None
pyqt5 : None

[graingert]

this was discovered in bokeh/bokeh#13057

[graingert]

looks like this was introduced in #50286

[phofl]

Yep, confirmed by bisect

b0305f7b8b58c36450ed4b4c285dcf8743c93f42 is the first bad commit
commit b0305f7b8b58c36450ed4b4c285dcf8743c93f42
Author: Patrick Hoefler <61934744+phofl@users.noreply.github.com>
Date:   Tue Dec 27 21:38:15 2022 +0100

    ENH: Add use_nullable_dtypes for read_html (#50286)

This was intentional. You'd have to update the default na values, if you want different behaviour here.
Are the docs sufficient?

[graingert]

Are the docs sufficient?

I now have a fix for bokeh bokeh/bokeh#13069 so I think so