Restrict topic reference lookups to classes

Currently, any call to `GetValue()` that doesn't match the other conditions—i.e., isn't a compatible or convertible property, isn't a list, and isn't `[MapToParent]`—is evaluated to determine if it's a topic reference. In practice, most properties that aren't topic references will have fallen into one of the other conditions. Nevertheless, as the lookups required for topic references are a bit expensive, we can further optimize this by specifying that the property type must be a class (i.e., not an interface and not a value object). That's consistent with what we expect topic references to be mapped to.

In practice, this will typically relate to potential properties that are non-compatible value properties. These are likely rare scenarios as they won't get mapped, and should probably be decorated with `[DisableMapping]`. Still, it's a fast check for refining the condition.

Author: JeremyCaney

OnTopic/Mapping/TopicMappingService.cs

@@ -541,7 +541,7 @@ await MapAsync(
       else if (configuration.Metadata.IsList) {
         return null;
       }
-      else if (associations.HasFlag(AssociationTypes.References)) {
+      else if (configuration.Metadata.Type.IsClass && associations.HasFlag(AssociationTypes.References)) {
         var topicReference = getTopicReference();
         if (topicReference is not null) {
           value = await GetTopicReferenceAsync(topicReference, targetType, configuration, cache).ConfigureAwait(false);