Gracefully fail if the TopicList cannot be resolved

Previously, if the `TopicListAttribute`'s `RelativeTopicBase` or `RootTopic` could not be resolved, then an error would end up getting thrown by `GetTopics()`. This adds in an escape condition so that an empty `List<QueryResultTopicViewmodel>` is returned if no `topic` is resolved.

Author: JeremyCaney

OnTopic.Editor.AspNetCore/Components/TopicListViewComponent.cs

@@ -158,6 +158,13 @@ public static List<QueryResultTopicViewModel> GetTopics(
       string allowedKeys        = ""
     ) {
 
+      /*------------------------------------------------------------------------------------------------------------------------
+      | Swallow missing topic
+      \-----------------------------------------------------------------------------------------------------------------------*/
+      if (topic == null) {
+        return new List<QueryResultTopicViewModel>();
+      }
+
       /*------------------------------------------------------------------------------------------------------------------------
       | Establish query options
       \-----------------------------------------------------------------------------------------------------------------------*/