[5.7] Improve loadMorph()

[staudenmeir]

loadMorph() has the same issue that #22363 fixed for load():

class Post extends Model {
    protected $with = ['user'];
}

$comments = Comment::with('commentable')->get()
    ->loadMorph('commentable', [
        Post::class => 'tags',
    ]);

When loadMorph() loads the tags relationship, it also loads the user relationship from $with.

We can fix it by just calling load() – which is also shorter.

We can also replace the filter() clause with a null coalescing operator.