Linux: shutil.which fails to find executable in current working directory/cwd when PATH unset (subprocess works)

Bug report

Bug description:

On Linux, executables from the cwd are used when PATH is unset:

$ PATH= python -V
Python 3.13.7

subprocess.run also works in this case:

$ PATH= python -q
>>> from subprocess import run
... run(['python', '-V'])
... 
Python 3.13.7
CompletedProcess(args=['python', '-V'], returncode=0)
>>> 

But shutil.which returns None:

>>> from shutil import which
>>> which('python')
>>> 

This is because when PATH is empty(""), the executable ("python") is passed to the kernel's execve directly, without resolving. execve then resolves the file relative to the cwd, I believe it's the case in both Linux and FreeBSD.
POSIX 2024 says this about the PATH environment variable:

If PATH is unset or is set to null, or if a path prefix in PATH contains a character ('%'), the path search is implementation-defined.

EDIT: PATH is empty, not unset

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Bắt đầu từ entry point shutil.which và tái hiện trường hợp Linux với PATH=, so sánh với subprocess.run(['python', '-V']). Xác định hành vi dự kiến đối với PATH rỗng, sau đó xác minh rằng shutil.which tuân theo hành vi đó và bao phủ trường hợp này bằng một regression test.