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>

Author: Nikita-Gz

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

@@ -158,7 +158,7 @@ async def send_activity(
  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: