Account for potentially nullable AttributeBindingModel.Value

With the `AttributeBindingModel.Value` now allowing null values (63e8b40), we need to account for that to prevent a null reference exception. With the `Key` setting, we assume a `Value` exists, as `Key` should always be `IsRequired`.

With relationships, we do the same thing, as while relationships aren't required, we expect an empty value if no relationships are being submitted; if that isn't true, that's a sign of a model binding issue, and should result in an exception.

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);