master: fix TypeError

Add a guard for the case `path == None`.

This commit fixes

`TypeError: stat: path should be string, bytes, os.PathLike or integer, not NoneType`

mitogen/master.py

@@ -462,7 +462,7 @@ class DefectivePython3xMainMethod(FinderMethod):
             return None
 
         path = getattr(mod, '__file__', None)
-        if not (os.path.exists(path) and _looks_like_script(path)):
+        if not (path is not None and os.path.exists(path) and _looks_like_script(path)):
             return None
 
         fp = open(path, 'rb')