BUG: pd.to_datetime with format doesn't work with pd.NA

[levkovalenko]

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

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

• (optional) I have confirmed this bug exists on the master branch of pandas.

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Note: Please read this guide detailing how to provide the necessary information for us to reproduce your bug.

Code Sample, a copy-pastable example

# Your code here
import numpy as np
import pandas as pd

a = pd.DataFrame({'a':[pd.NA]})  
b = pd.DataFrame({'b': [np.nan]})
pd.to_datetime(b['b'])                              # -> Work
pd.to_datetime(b['b'], format="%Y%m%d%H%M%S")       # -> Work
pd.to_datetime(a['a'])                              # -> Work
pd.to_datetime(a['a'], format="%Y%m%d%H%M%S")       # -> Error

Problem description

I try to use pd.NA to define NaN values in field what contains datetime strings.
I try convert them to datetime, but it doesn't work.
I get error ValueError: time data '<NA>' does not match format '%Y%m%d%H%M%S' (match)

Actually the same code with np.nan works correctly

Expected Output

pd.NaT with no errors

Output of pd.show_versions()

INSTALLED VERSIONS

commit : c7f7443
python : 3.9.6.final.0
python-bits : 64
OS : Linux
OS-release : 4.18.0-305.7.1.el8_4.x86_64
Version : #1 SMP Mon Jun 14 17:25:42 EDT 2021
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : C.utf8
LOCALE : en_US.UTF-8

pandas : 1.3.1
numpy : 1.21.1
pytz : 2021.1
dateutil : 2.8.2
pip : 21.2.2
setuptools : 49.6.0.post20210108
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : None
jinja2 : 3.0.1
IPython : 7.25.0
pandas_datareader: None
bs4 : 4.9.3
bottleneck : 1.3.2
fsspec : 2021.07.0
fastparquet : None
gcsfs : None
matplotlib : 3.4.2
numexpr : 2.7.3
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyxlsb : None
s3fs : None
scipy : 1.7.0
sqlalchemy : None
tables : 3.6.1
tabulate : 0.8.9
xarray : None
xlrd : None
xlwt : None
numba : 0.53.1

[debnathshoham]

take

[levkovalenko]

@debnathshoham

Hi, i see your PR

It doesn't fix BUG. I'm sorry for bad desription, but i have bug with this code:

import numpy as np
import pandas as pd

a = pd.DataFrame({'a':[pd.NA, "20210202202020"]})  
b = pd.DataFrame({'b': [np.nan, "20210202202020"]})
print(pd.to_datetime(b['b'], format="%Y%m%d%H%M%S"))      # -> Work
# 0                   NaT
# 1   2021-02-02 20:20:20
# Name: b, dtype: datetime64[ns]
print(pd.to_datetime(a['a'], format="%Y%m%d%H%M%S"))       # -> Error
#ValueError: time data '<NA>' does not match format '%Y%m%d%H%M%S' (match)

So datetime formatter can't work with pd.NA values but for np.nan everything is good.