Domain Model check uses __bool_ rather than 'is not None'

[jgrey4296]

If you provide a domain model with a __bool__ method, that evaluates to False when constructing a a statemachine, the logic falls back to use the default model. If there are domain model conditions, then the StateMachine instantiation fails with an error like:

statemachine.exceptions.InvalidDefinition: Error on transition setup progress from Named to Dead when resolving callbacks: not found name 'spec_missing' from model or statemachine

Which doesn't provide any information to say the Domain Model is the default domain rather than the expected domain.

Probably, a StateMachine's __init__ should use model if model is not None else model() rather than the implicit bool, or missing conditions/validations/callback errors should specify the type of the domain model so the user can see their model isn't being used.

Relevant line:

python-statemachine/statemachine/statemachine.py

Line 78 in b0367f0

self.model = model if model else Model()