fix attachment_action inputs property

Author: Jian Chen

webexteamssdk/models/mixins/attachment_action.py

@@ -70,7 +70,7 @@ def messageId(self):
     @property
     def inputs(self):
         """The attachment action's inputs"""
-        return self._json_data('inputs')
+        return self._json_data.get('inputs')
 
     @property
     def created(self):