[Bug 45320] New: cmd creates an environment variable with an empty name

https://bugs.winehq.org/show_bug.cgi?id=45320

Bug ID: 45320
           Summary: cmd creates an environment variable with an empty name
           Product: Wine
           Version: 3.9
          Hardware: x86-64
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: cmd
          Assignee: [e-mail]
          Reporter: [e-mail]
      Distribution: ---

CMD creates an environment variable with an empty name when it starts up. That
doesn't match the behavior on Windows (at least, Win10) and breaks programs
like Python.

Apparently, CMD is trying to save the current working directory; the code was
added in
https://source.winehq.org/git/wine.git/commit/d0db751e0cb7a0526c3aecca489189...,
but without an explanation.

To reproduce, run this:

#include <stdio.h>

int main(int argc, char **argv, char **envp) {
  for (char **env = envp; *env != 0; env++) {
    printf("%s\n", *env);
  }
  return 0;
}

The last line of the output will be similar to this:

=Z:=Z:\home\dima

Python gets an exception if it tries to unset that variable:
> > > import os
> > > os.environ.clear()

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\ProgramData\Miniconda3\lib_collections_abc.py", line 820, in clear
    self.popitem()
  File "C:\ProgramData\Miniconda3\lib_collections_abc.py", line 813, in
popitem
    del self[key]
  File "C:\ProgramData\Miniconda3\lib\os.py", line 680, in __delitem__
    self.unsetenv(encodedkey)
  File "C:\ProgramData\Miniconda3\lib\os.py", line 718, in <lambda>
    _unsetenv = lambda key: _putenv(key, "")
OSError: [Errno 0] Error

This is arguably a Python bug, and it happens on UNIX, too:
https://bugs.python.org/issue20658 . However, cmd should probably match the
Windows behavior and not trigger the Python bug unnecessarily.