Default to ValueProperty of Key

The previous code assumed that `ValueProperty` is always set for `TopicListAttribute`s. That shouldn't be assumed. If the `ValueProperty` isn't set, for some reason, we should fall back to a reasonable default. `Key` makes sense in most scenarios.

Author: JeremyCaney

OnTopic.Editor.AspNetCore/Components/TopicListViewComponent.cs

@@ -58,7 +58,7 @@ public IViewComponentResult Invoke(
       attribute.RootTopicKey    ??= attribute.GetConfigurationValue(            "Scope",                null);
       attribute.AttributeKey    ??= attribute.GetConfigurationValue(            "AttributeName",        null);
       attribute.AttributeValue  ??= attribute.GetConfigurationValue(            "AttributeValue",       null);
-      attribute.ValueProperty   ??= attribute.GetConfigurationValue(            "ValueProperty",        null);
+      attribute.ValueProperty   ??= attribute.GetConfigurationValue(            "ValueProperty",        "Key");
       var       allowedKeys       = attribute.GetConfigurationValue(            "AllowedKeys",          null);
 
       /*------------------------------------------------------------------------------------------------------------------------