`Spond.get_event()` and `Spond.get_group()` fail with unexpected `TypeError` if no events/groups are available

[elliot-100]

get_event() and get_group() call get_events() and get_groups(), which return None if no events/groups are available.

(Until I looked at it properly, I thought they returned empty lists.)

This appears to cause problems:

>       await s.get_event("ID1")

...
        if not self.events:
            await self.get_events()
        for event in self.events:
>           if event["id"] == uid:
E           TypeError: string indices must be integers

..\spond\spond.py:312: TypeError

I'm looking into this.

[elliot-100]

On closer inspection, what actually happens in above is that the JSON response is an authentication error from the server, which parses as a dict instead of expected list[dict].

Thus the error message: event in this case is actually a dict key str instead of expected dict.

I will look at the authentication code but I have to say I don't really understand it - it seems to handle AuthenticationError in two places, for example. I am not sure that mock_require_authentication() in tests actually works properly, either...

[Edit to add: the code as designed should be able to return None if no events/groups are available, so PR #151 to cover #147 is still valid... I think]