Nullable polymorphic relationship combined with $touches generates incorrect query

[jerguslejko]

• Laravel Version: 5.6.37
• PHP Version: 7.2.9
• Database Driver & Version: mysql & 5.7.23

Description:

When using an inverse of a nullable polymorphic (morphTo()) relationship along side with $touches on the relationship, Laravel generates incorrect query for touching the relationship.

Steps To Reproduce:

1. Create tags table with nullable taggable fields:

   Schema::create('tags', function (Blueprint $table) {
       $table->increments('id');
   
       $table->nullableMorphs('taggable');
   
       $table->timestamps();
   });

2. Create Tag model with taggable() relationship and $touches property set to ['taggable']

   class Tag extends Model
   {  
       protected $touches = ['taggable'];
   
       public function taggable()
       {
           return $this->morphTo();
       }
   }

3. Now try to create a new Tag

   Tag::create();

This generates the following error:

Illuminate/Database/QueryException with message 'SQLSTATE[42S22]: Column not found: 1054 Unknown column 'tags.' in 'where clause' (SQL: update `tags` set `updated_at` = 2018-09-03 20:31:36 where `tags`.`` is null)'

Thanks for looking into this