Fix disable autofill issue

[joshhanley]

The scenario

Currently if you have a property that has a StateId set to null, Verbs will generate an ID for it.

But there are some scenarios where a property should actually be set to null and no new ID/ state should get generated for it, as it's an optional reference to another state.

To solve this, we can use the autofill: false property on the StateId attribute.

#[StateId(StateIdTestState::class, autofill: false)]
public ?int $other_state_id = null;

The problem

The problem is that at the moment, doing the above code throws the following error:

The solution

This PR fixes it by ensuring that when the EventStateRegistry discovers states, that it has actually found a state before trying to process the aliases.

I couldn't see any unit tests for StateId so I have added a basic test and added 2 tests that cover the autofill: false scenarios.