Resolve inadvertent overwrite of Value, InheritedValue

In #e88dee0, I resolved a bug where the optional `Value` and `InheritedValue` constructor parameters weren't being processed. This fixed that issue, but resolved a new one: The default inherited values used when these were null, as assigned by the base `AttributeViewModel` constructor, were being overwritten. Whoops. This is fixed by checking them for nulls.

Author: JeremyCaney

OnTopic.Editor.Models/AttributeViewModel{T}.cs

@@ -32,8 +32,8 @@ public AttributeViewModel(
       attributeDescriptor
     ) {
       AttributeDescriptor       = attributeDescriptor;
-      Value                     = value;
-      InheritedValue            = inheritedValue;
+      Value                     = value?? Value;
+      InheritedValue            = inheritedValue?? InheritedValue;
     }
 
     /*==========================================================================================================================