better check for cwd being empty

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Author: kulikjak

Lib/subprocess.py

@@ -1785,7 +1785,7 @@ def _posix_spawn(self, args, executable, env, restore_signals, close_fds, cwd,
                 if fd != -1:
                     file_actions.append((os.POSIX_SPAWN_DUP2, fd, fd2))
 
-            if cwd:
+            if cwd is not None:
                 file_actions.append((os.POSIX_SPAWN_CHDIR, cwd))
 
             if close_fds: