fix(Harness): remove special-casing for get_relation in leader-elected

[tonyandrewmeyer]

In Juju, using get_relation() in a leader-elected event will return None if the relation does not yet exist. This makes Harness consistent with that behaviour.

This does remove a safety-check for badly written tests - where you mean for the relation to exist but forget to do an add_relation() call first in the test. However, we don't have any sort of check like this for any other events, or any other types of relations, and it should be obvious when the test fails that it's failing to find an expected relation, and the solution is to set that relation up. I don't think it's worth having a warning for this specific case (we don't warn in other similar situations).

This also means we can remove a check in model.py that's a no-op in production and does work in Harness, which is a bad smell, in my opinion.

Done:

• Check with Juju team whether peer-relations are actually guaranteed to be set up before leader-elected
• Quick super-tox check to make sure that no-one is somehow relying on this

Fixes #1153

[tonyandrewmeyer]

• Check with Juju team whether peer-relations are actually guaranteed to be set up before leader-elected

The answer is that this is current behaviour, not guaranteed behaviour. So it's extra bad for ops (Harness) to behave like it's guaranteed.