Fix Owner Morph Class Resolution and Add Check for 0 in Morph Map Keys

[rbondoc96]

Because of the changes made in this commit, the output of getMorphClass on the polymorphic owner is incompatible with the audit_changes.owner_type and audit_models.owner_type columns.

This PR also adds error checking against morph maps with a 0-key, as they ultimately get resolved to false deep in Laravel's relationship logic, which prevents models with that key from being morphed properly.

[Jasonej]

Just to clarify on this for anyone interested: It's specifically the buildDictionary logic that is used when eager loading.

    protected function buildDictionary(Collection $models)
    {
        foreach ($models as $model) {
            if ($model->{$this->morphType}) {
                $morphTypeKey = $this->getDictionaryKey($model->{$this->morphType});
                $foreignKeyKey = $this->getDictionaryKey($model->{$this->foreignKey});

                $this->dictionary[$morphTypeKey][$foreignKeyKey][] = $model;
            }
        }
    }

The if ($model->{$this->morphType}) { check is what get resolved to false when the morph map value is a 0. This prevents the model from being added to dictionary and ultimately results in attempts at eager loading relationships such as entity always returning null when the relationship is to the model with a morph map value of 0.