[5.6] Remove null relationships in loadMorph before grouped by class name

[beamjet]

Relationship model may be null when using with() to eager load them, which will cause error in loadMorph() because of the usage of get_class().
So, remove null relationships before group them by class name seems reasonable.

[browner12]

could you please provide a more thorough description and use case of what this fixes?

also, probably needs to go to 5.7

[tillkruss]

Please use filter() without a callback.

[beamjet]

@tillkruss Thanks for your review. Use filter() without a callback is just much more concise.

[beamjet]

@browner12 Here is a example:

$notifications = Auth::user()->notifications()
            ->with('subject')
            ->get()
            ->loadMorph('subject', [
                EventInvite::class => 'event',
                GroupPhotoUpload::class => 'group',
                GroupPost::class => 'group',
                PostLike::class => 'post',
            ]);

In this case, subject relationship of notifications are eager-loaded. But some notifications may have no subject relationship and subject model will be null in this case, which will cause error in loadMorph because it passes null to get_class().