BUG: astype("str") converts NA values to strings

[alexreg]

• 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 master branch of pandas.

Reproducible Example

import pandas as pd
import numpy as np

pd.Series(["foo", "bar", pd.NA]).astype("str") // ["foo", "bar", "<NA>"]
pd.Series(["foo", "bar", np.NaN]).astype("str") // ["foo", "bar", "nan"]
pd.Series(["foo", "bar", None]).astype("str") // ["foo", "bar", "None"]

Issue Description

NA/null values are converted to their string representation when doing .astype("str"), which seems like odd/undesirable behaviour.

Expected Behavior

import pandas as pd
import numpy as np

pd.Series(["foo", "bar", pd.NA]).astype("str") // ["foo", "bar", pd.NA]
pd.Series(["foo", "bar", np.NaN]).astype("str") // ["foo", "bar", np.NaN]
pd.Series(["foo", "bar", None]).astype("str") // ["foo", "bar", None]

Installed Versions

INSTALLED VERSIONS

commit : 9b2bb73
python : 3.9.7.final.0
python-bits : 64
OS : Darwin
OS-release : 20.6.0
Version : Darwin Kernel Version 20.6.0: Wed Jun 23 00:26:31 PDT 2021; root:xnu-7195.141.2~5/RELEASE_X86_64
machine : x86_64
processor : i386
byteorder : little
LC_ALL : None
LANG : en_GB.UTF-8
LOCALE : en_GB.UTF-8

pandas : 1.4.0.dev0+952.g9b2bb732f0
numpy : 1.21.3
pytz : 2021.3
dateutil : 2.8.2
pip : 21.3
setuptools : 58.2.0
Cython : 0.29.24
pytest : 6.2.5
hypothesis : 6.23.4
sphinx : 4.2.0
blosc : None
feather : None
xlsxwriter : None
lxml.etree : 4.6.3
html5lib : 1.1
pymysql : None
psycopg2 : None
jinja2 : 3.0.2
IPython : None
pandas_datareader: None
bs4 : 4.10.0
bottleneck : None
fsspec : None
fastparquet : None
gcsfs : None
matplotlib : None
numexpr : None
odfpy : None
openpyxl : 3.0.9
pandas_gbq : None
pyarrow : 5.0.0
pyxlsb : None
s3fs : None
scipy : None
sqlalchemy : None
tables : None
tabulate : 0.8.9
xarray : None
xlrd : 2.0.1
xlwt : None
numba : None

[jreback]

there is an open PR that addresses pls search for the issue and link it

[alexreg]

@jreback Is this the one? #41412

I don't have privileges to actually link the issue to the PR, as far as I'm aware.

[jreback]

@alexreg thanks now
does this duplicate #40839 ?

[alexreg]

@jreback I believe so, but I'm by no means 100% sure. Maybe best to leave this open for now so the PR author can add a test case for this issue too? If that's alright with you...

[asishm]

duplicate of #25353

also #37034 introduced tests that explicitly test this behavior