Result of os.path.realpath("") is incorrect if cwd is on letter-less drive (\\?\-prefixed)

Bug report

1. Create volume, dir on it, don't assign a drive letter
2. In Python:

>>> import os
>>> os.chdir("\\\\?\\Volume{<uuid>}\\<dir>")
>>> os.path.realpath("")
'\\\\?\\Volume{<uuid>}\\<dir>\\.'

The dot is invalid in \\?\-prefixed path.
Dots (. and ..) are normalized by Windows API (at least on NTFS). But \\?\ paths bypass normalization and so may not contain dots. See, for example, here.
All later WinAPI calls with this path fail.

As a simple test, dir \\?\Volume{<uuid>}\dir\. doesn't work whereas dir \\?\Volume{<uuid>}\dir works.

P.S. os.path.realpath(".") gives the same invalid result

Your environment

CPython 3.10.0
Windows 10

Context
The problem arises from pip.
sys.path contains empty string (meaning current dir I presume).
And legacy installation method resolves sys.path relatively to cwd.

Investigation
Here:
https://github.com/python/cpython/blob/4bed0db7c222f8df1b4e31107c0305214caf3f56/Lib/ntpath.py#L704
os.path.join still adds slash even if the second argument is empty. And if the path is ended with slash, dot is added somewhere below.

I fixed it to:

path = cwd if path == "." or path == "" else join(cwd, path)

Inizia in Lib/ntpath.py intorno alla riga 704 indicata e riproduci il comportamento di Windows con un percorso di volume \?-con prefisso la cui directory corrente non ha una lettera di unità. Verifica come os.path.realpath("") e os.path.realpath(".") costruiscono i relativi percorsi, quindi aggiungi una copertura di regressione che mostri che nessuno dei due risultati termina con un componente \. non valido e che il percorso risultante funziona con le chiamate WinAPI successive.