fix issue with event looper

[ziyadm]

Task exception was never retrieved future: <Task finished name='Task-14' coro=<EventHandler.enforce_time_limits_loop() done, defined at /Users/ziyadm/Desktop/code/test-bot-repo/event_handler.py:13> exception=KeyError('discord_channel_id')> Traceback (most recent call last): File "/Users/ziyadm/Desktop/code/test-bot-repo/event_handler.py", line 15, in enforce_time_limitsloop = await self.state.enforce_time_limits() File "/Users/ziyadm/Desktop/code/test-bot-repo/state.py", line 177, in enforce_time_limits all_missions = await Mission.rows(formula=None, airtable_client=self.airtable_client) File "/Users/ziyadm/Desktop/code/test-bot-repo/mission.py", line 146, in rows return [cls.of_airtable_response(response) for response in responses] File "/Users/ziyadm/Desktop/code/test-bot-repo/mission.py", line 146, in return [cls.of_airtable_response(response) for response in responses] File "/Users/ziyadm/Desktop/code/test-bot-repo/mission.py", line 129, in of_airtable_response return cls(record_id=response.record_id, fields=Fields.of_dict(response.fields)) File "/Users/ziyadm/Desktop/code/test-bot-repo/mission.py", line 88, in of_dict discord_channel_id=fields[cls.discord_channel_id_field], KeyError: 'discord_channel_id'

[ziyadm]

this happens when airtable has empty rows in the missions table - shouldn't be a problem