Close #12383: Fix subprocess module with env={}: don't copy the environment

variables, start with an empty environment.

Lib/subprocess.py

@@ -1169,7 +1169,7 @@ class Popen(object):
                         # potential deadlocks, thus we do all this here.
                         # and pass it to fork_exec()
 
-                        if env:
+                        if env is not None:
                             env_list = [os.fsencode(k) + b'=' + os.fsencode(v)
                                         for k, v in env.items()]
                         else: