This pull request fixes errors when 1 model has multiple polymorphic relationships sharing same "type" column.

In case we have a polymorphic relationship, where _type column is empty, script will work great. But if the _id column is empty, script will throw an exception.

For an example, if model has the following columns:

• related_type (non-nullable)
• related_old_id (nullable)
• related_new_id (nullable)

and the following relationships:

    public function oldRelatedModel(): MorphTo
    {
        return $this->morphTo(name: null, type: 'related_type', id: 'related_old_id');
    }

    public function newRelatedModel(): MorphTo
    {
        return $this->morphTo(name: null, type: 'related_type', id: 'related_new_id');
    }

When we eager load both polymorphic relationships and one of the columns (related_old_id or related_new_id) are null, execution will fail (especially if we are dealing with uuids) because the function will convert all values to string (null => '') and '' is invalid uuid syntax.

ERROR: invalid input syntax for type uuid: \"\" (SQL: select * from \"user\" where \"user\".\"id\" in (, 68504945-959d-4e13-b1b4-7a250523cc24))"

This pull requests filters out nullable values and eager loads only non-nullable values.