[12.x] Lazily evaluate value for constraints in HasOneOrManyThrough

[Jacobs63]

In a scenario, where:

1. Primary keys are casted, in our case we're using value objects representing an integer with a value >= 1 called PositiveInteger
2. Using a relation that extends HasOneOrManyThrough results in an exception

The reason is, that at the moment of calling addConstraints in the context of constructor of either HasOneThrough or HasManyThrough, the $farParent is empty.
This means our cast, which is highly strict, results in an exception.

When debugging I've noticed that, unlike any other relation's addContraints method, this is for some (unknown) reason evaluated even if the static::$constraints is set to false which, coincidentally, breaks our application.

This PR changes the behaviour in \Illuminate\Database\Eloquent\Relations\HasOneOrManyThrough::addConstraints to resolve the value for constraint in the if statement, therefore fixing this inconsistency (and our bug).

[Jacobs63]

The failing tests seem to be unrelated to this PR, as they're also failing in the 12.x branch.