Consider all model fields for model truthiness checks

[lu-pl]

Depends on #110 .

Currently the model serialization values are checked in order to determine model truthiness.

If excluded model fields should be applicable for determining model truthiness according to #110, the default check must not rely on model serialization because excluded fields obviously will be - well - excluded. See #110 (comment).

A solution for this is to use model dict-casting instead of model_dump in the default model_bool predicate.

The default model_bool predicate would then have to be this:

class ModelBoolPredicate(Protocol):
    def __call__(self, model: _TModelInstance) -> bool: ...

def default_model_bool_predicate(model: _TModelType) -> bool: 
    return any(dict(model).values())