[5.7] Fix nullable MorphTo touching

[staudenmeir]

MorphTo has its own touch() implementation to handle nullable related models:

Schema::create('comments', function ($table) {
    $table->increments('id');
    $table->nullableMorphs('commentable');
});

class Comment extends Model {
    protected $touches = ['commentable'];

    public function commentable() {
        return $this->morphTo();
    }
}

Comment::create();

However, this doesn't cover relationships with a custom $ownerKey:

class Comment extends Model {
    protected $touches = ['commentable'];

    public function commentable() {
        return $this->morphTo(null, null, null, 'id');
    }
}

We have to check the actual parent key instead of the $ownerKey.