BUG: pd.to_datetime with format doesn't work with pd.NA
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

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**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
# 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()``

<details>

INSTALLED VERSIONS
------------------
commit           : c7f7443c1bad8262358114d5e88cd9c8a308e8aa
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

</details>
