Add the violation only if the entity has the unique title enabled

I found this patch is causing errors when try to access to get the value when we create a new content. Before we access to [0] it is needed to check item has a value. As part of it I have also ensure we never pass a null value to isNotUnique due to that's causing more issues with php8.1.

MR#3 works well for me. Thanks.

Works as expected on Drupal Version 9.5.10 with unique_entity_title 1.3.0

Thanks!

Automatically closed - issue fixed for 2 weeks with no activity.