fix: fix the corner case when variable is None

Sometimes application developers set the value of an environment
variable to None. This fix adds a guard for mitigating this issue to
prevent python from crashing.

Signed-off-by: Tianrui Wei <tianrui@tianruiwei.com>

Author: tianrui-wei

Lib/subprocess.py

@@ -1815,6 +1815,8 @@ def _execute_child(self, args, executable, preexec_fn, close_fds,
                     if env is not None:
                         env_list = []
                         for k, v in env.items():
+                            if v is None:
+                                continue
                             k = os.fsencode(k)
                             if b'=' in k:
                                 raise ValueError("illegal environment variable name")