Prefer is over == when working with constant values

This was previously applied to most scenarios (e6bd2b7), but missed in this scenario.

Author: JeremyCaney

OnTopic/Mapping/TopicMappingService.cs

@@ -339,7 +339,7 @@ private async Task<object> MapAsync(
       if (cache.TryGetValue(topic.Id, target.GetType(), out var cacheEntry)) {
         associations            = cacheEntry.GetMissingAssociations(associations);
         target                  = cacheEntry.MappedTopic;
-        if (associations == AssociationTypes.None) {
+        if (associations is AssociationTypes.None) {
           return cacheEntry.MappedTopic;
         }
         cacheEntry.AddMissingAssociations(associations);