Merge branch 'bugfix/AttributeBindingModel.Value-required' into release/5.1.0

In a previous update, we changed the syntax for non-nullable properties to use the null-forgiving operator instead of the nullable attributes (e.g., `[NotNull]`, `[DisallowNull]`). These should be equivalent, but the ASP.NET Core model validation syntax doesn't treat them as such. As a result, it started throwing model validation exceptions for `Value` when a radio button wasn't selected, in which case no value is set. Technically, this should have been a long-standing bug; the new behavior is actually correct. This exposes an issue that the `Value` _should_ be nullable, for the specific case of radio buttons.

By marking this nullable, some downstream calls in `EditorController` raised potential null dereference warnings. In both cases, we would never expect the value to be null, since those types (`Key`, relationships) don't use radio buttons. As such, these were both decorated with the null-forgiving operator as well; if they are _not_ null, a dereference exception _should_ occur—though we never expect this to occur, so we're not _explicitly_ checking for this scenario.

Author: JeremyCaney

OnTopic.Editor.AspNetCore/Controllers/EditorController.cs

@@ -438,7 +438,7 @@ public async Task<IActionResult> Edit(
           SetReference(topic, attribute, attributeValue);
         }
         else if (attribute.Key is "Key") {
-          topic.Key = attributeValue.Value.Replace(" ", "", StringComparison.Ordinal);
+          topic.Key = attributeValue.Value!.Replace(" ", "", StringComparison.Ordinal);
         }
         else if (topic.BaseTopic is null && String.IsNullOrEmpty(attributeValue.Value)) {
           topic.Attributes.SetValue(attribute.Key, attribute.DefaultValue);
@@ -471,7 +471,7 @@ public async Task<IActionResult> Edit(
     ///   Private helper function that saves relationship values to the topic.
     /// </summary>
     private void SetRelationships(Topic topic, AttributeDescriptor attribute, AttributeBindingModel attributeValue) {
-      var relatedTopics = attributeValue.Value.Split(',', StringSplitOptions.RemoveEmptyEntries).ToList();
+      var relatedTopics = attributeValue.Value!.Split(',', StringSplitOptions.RemoveEmptyEntries).ToList();
       topic.Relationships.Clear(attribute.Key);
       foreach (var topicIdString in relatedTopics) {
         Topic? relatedTopic = GetAssociatedTopic(topicIdString);

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