Mitigated validation bug with recent Move() bug fix

When fixing the missing implementation of the basic `Move()` overload (d7a61cd), I failed to take into account that `SetParent()` validates that the sibling is not null. Perhaps we should align the logic of `SetParent()` to behave like `Move()` where `sibling` can be `null`. In the meanwhile, however, it's easy to workaround.

Author: JeremyCaney

OnTopic/Repositories/TopicRepositoryBase.cs

@@ -423,7 +423,12 @@ public virtual void Move([ValidatedNotNull]Topic topic, [ValidatedNotNull]Topic
       \-----------------------------------------------------------------------------------------------------------------------*/
       var previousParent        = topic.Parent;
       MoveEvent?.Invoke(this, new MoveEventArgs(topic, target));
-      topic.SetParent(target, sibling);
+      if (sibling == null) {
+        topic.SetParent(target);
+      }
+      else {
+        topic.SetParent(target, sibling);
+      }
 
       /*------------------------------------------------------------------------------------------------------------------------
       | If a content type descriptor is being moved to a new parent, refresh cache