Improve overloads of parse_args() -- it never returns None

Fixes #2641

Author: Guido van Rossum

stdlib/2and3/argparse.pyi

@@ -125,8 +125,12 @@ class ArgumentParser(_AttributeHolder, _ActionsContainer):
     @overload
     def parse_args(self, args: Optional[Sequence[_Text]] = ...) -> Namespace: ...
     @overload
+    def parse_args(self, args: Optional[Sequence[_Text]], namespace: None) -> Namespace: ...
+    @overload
     def parse_args(self, args: Optional[Sequence[_Text]], namespace: _N) -> _N: ...
     @overload
+    def parse_args(self, *, namespace: None) -> Namespace: ...
+    @overload
     def parse_args(self, *, namespace: _N) -> _N: ...
 
     if sys.version_info >= (3, 7):