Updated AttributeBindingModel.Value to allow null values

In a previous update, I switched from `[NotNull, DisallowNull]` to `= default!`. These _should_ be syntactically equivalent. But they're not actually treated that way by the model binding validator, which will allow `[DisallowNull]` on a nullable type, but not on a non-nullable type. This is an issue because radio buttons, in particular, might be null—which is why we don't validate `Value` in the `AttributeBindingModelBinder`.

This can be fixed by making `Value` nullable. This has some downstream effects, which will be addressed separately.

Author: JeremyCaney

OnTopic.Editor.AspNetCore/Models/AttributeBindingModel.cs

@@ -58,7 +58,7 @@ public AttributeBindingModel(string editorType) {
     /// <summary>
     ///   The value associated with the attribute.
     /// </summary>
-    public string Value { get; init; } = default!;
+    public string? Value { get; init; } = default!;
 
     /*==========================================================================================================================
     | GET VALUE
@@ -71,7 +71,7 @@ public AttributeBindingModel(string editorType) {
     ///   <see cref="GetValue()"/> method is intended to be overwritten by derived versions of the <see cref="AttributeBindingModel"/>
     ///   class, in order to provide specific serialization instructions.
     /// </remarks>
-    public virtual string GetValue() => Value;
+    public virtual string GetValue() => Value?? "";
 
   } // Class
 } // Namespace