[5.6] Fix nullable MorphTo and $touches

[staudenmeir]

Using $touches with a MorphTo relationships breaks without a related model:

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();

This throws an exception:

Unknown column 'comments.' in 'where clause'
(SQL: update comments set updated_at = 2018-09-04 02:24:54 where comments.`` is null)

We can detect this case by checking the $ownerKey and so prevent the invalid query.

Fixes #25434.

[jerguslejko]

I can confirm that this fixes #25434 👍