Added test & update to prevent page creation w/ empty slug

Caused by changes to page repo in reference work,
This adds back in the slug generate although at a more central place.
Adds a test case to cover the problematic scenario.

app/Entities/Repos/BaseRepo.php

@@ -56,7 +56,7 @@ class BaseRepo
         $entity->fill($input);
         $entity->updated_by = user()->id;
 
-        if ($entity->isDirty('name')) {
+        if ($entity->isDirty('name') || empty($entity->slug)) {
             $entity->refreshSlug();
         }