Fixed type hint in turn_context.py (#2147)

Previous type hint implied that send_activity will always return ResourceResponse, meanwhile it can also return None

Co-authored-by: tracyboehrer <tracyboehrer@users.noreply.github.com>

libraries/botbuilder-core/botbuilder/core/turn_context.py

@@ -158,7 +158,7 @@ class TurnContext:
         activity_or_text: Union[Activity, str],
         speak: str = None,
         input_hint: str = None,
-    ) -> ResourceResponse:
+    ) -> Union[ResourceResponse, None]:
         """
         Sends a single activity or message to the user.
         :param activity_or_text: