[5.7] Prevent unnecessary relationship queries

[staudenmeir]

Prevents unnecessary queries when lazy loading empty relationships:

(new Post(['user_id' => null]))->user; // null
(new User)->posts;                     // []

MorphTo::getResults() is now handled by BelongsTo::getResults() (#8586).

This does not apply to custom relationship queries like (new User)->posts()->get().

I don't think we need additional tests for this. There are plenty of tests that verify relationship queries.

Fixes #25214, #24241, #20616 and laravel/ideas#748.

[brunogaspar]

Good one!