Count cache support nullable

[fmarquesto]

When we use CountedBy on a nullable field and we set this to null:

...
$table->integer('user_id')->nullable();
...

$post->userId = null;

We receive an unexpected error: TypeError : Eloquence\Behaviours\CacheConfig::relatedModel(): Return value must be of type Illuminate\Database\Eloquent\Model, null returned

With this PR I've added a validation to avoid retrieving the model on the update method if the foreignKey value is null.