botserver: Fix mypy error.

Value of type "Optional[Any]" is not indexable error
was originated in PR zulip#682.

zulip_botserver/zulip_botserver/server.py

@@ -169,6 +169,7 @@ def init_message_handlers(
 @app.route('/', methods=['POST'])
 def handle_bot() -> str:
     event = request.get_json(force=True)
+    assert event is not None
     for bot_name, config in bots_config.items():
         if config['email'] == event['bot_email']:
             bot = bot_name