BUG: _guess_datetime_format_for_array doesn't guess if first element is '' or 'NaT'

[MarcoGorelli]

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• 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

from pandas.core.tools.datetimes import _guess_datetime_format_for_array
print(_guess_datetime_format_for_array(['NaT', '13-01-2000']))  # prints None
print(_guess_datetime_format_for_array(['', '13-01-2000']))  # prints None

Issue Description

Both 'NaT' and '' end up becoming NaT in tslib.array_to_datetime, so they should be ignored when finding the first non-null element to use to guess the datetime format.

However, currently that doesn't happen, because

pandas/pandas/core/tools/datetimes.py

Lines 129 to 131 in 0106c26

	non_nan_elements = notna(arr).nonzero()[0]
	if len(non_nan_elements):
	if type(first_non_nan_element := arr[non_nan_elements[0]]) is str:

doesn't exclude 'NaT' or '' when looking for the first non-null element

Expected Behavior

to get

%d-%m-%Y
%d-%m-%Y

Installed Versions

INSTALLED VERSIONS

commit : 16d6e095b9cfb7ca33f47a6cdbcbc8f61fd112dc
python : 3.8.13.final.0
python-bits : 64
OS : Linux
OS-release : 5.10.102.1-microsoft-standard-WSL2
Version : #1 SMP Wed Mar 2 00:30:59 UTC 2022
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_GB.UTF-8
LOCALE : en_GB.UTF-8

pandas : 1.6.0.dev0+346.g16d6e095b9
numpy : 1.23.3
pytz : 2022.2.1
dateutil : 2.8.2
setuptools : 59.8.0
pip : 22.2.2
Cython : 0.29.32
pytest : 7.1.3
hypothesis : 6.54.6
sphinx : 4.5.0
blosc : None
feather : None
xlsxwriter : 3.0.3
lxml.etree : 4.9.1
html5lib : 1.1
pymysql : 1.0.2
psycopg2 : 2.9.3
jinja2 : 3.0.3
IPython : 8.5.0
pandas_datareader: 0.10.0
bs4 : 4.11.1
bottleneck : 1.3.5
brotli :
fastparquet : 0.8.3
fsspec : 2021.11.0
gcsfs : 2021.11.0
matplotlib : 3.6.0
numba : 0.56.2
numexpr : 2.8.3
odfpy : None
openpyxl : 3.0.10
pandas_gbq : 0.17.8
pyarrow : 9.0.0
pyreadstat : 1.1.9
pyxlsb : 1.0.9
s3fs : 2021.11.0
scipy : 1.9.1
snappy :
sqlalchemy : 1.4.41
tables : 3.7.0
tabulate : 0.8.10
xarray : 2022.9.0
xlrd : 2.0.1
xlwt : 1.3.0
zstandard : 0.18.0
tzdata : None
None