Removed unnecessary null check

The method is nullable, so if `topicDto` is null, then the `return topicDto` will return null. This is entirely unnecessary.

Author: JeremyCaney

OnTopic/Mapping/TopicMappingService.cs

@@ -1050,13 +1050,6 @@ MappedTopicCache cache
         topicDto = await MapAsync(source, mappingType, configuration.IncludeAssociations, cache).ConfigureAwait(false);
       }
 
-      /*------------------------------------------------------------------------------------------------------------------------
-      | Validate results
-      \-----------------------------------------------------------------------------------------------------------------------*/
-      if (topicDto is null) {
-        return null;
-      }
-
       /*------------------------------------------------------------------------------------------------------------------------
       | Return type
       \-----------------------------------------------------------------------------------------------------------------------*/