[8.x] Stop throwing LazyLoadingViolationException for recently created model instances

[rogatty]

Fixes #41420. I think the issue describes the problem and the solution pretty well.

In addition, I had to make a change in Model::handleLazyLoadingViolationUsing which now accepts null, so tests can reset the state. Class property was already nullable so I think it's a good step anyway:

framework/src/Illuminate/Database/Eloquent/Model.php

Lines 170 to 175 in dd59f62

	/**
	* The callback that is responsible for handling lazy loading violations.
	*
	* @var callable|null
	*/
	protected static $lazyLoadingViolationCallback;

At first, I was surprised that new tests were passing without any changes in the code. Then I figured out that only a local public $preventsLazyLoading = true; causes the issue, while global Model::preventLazyLoading() does not. It seems the latter one doesn't trigger the check when using make() or create(). I'd prefer not to dive into this topic now, thus the new class in tests.