Ensure labels aren't treated as values

The `TopicListViewComponent` includes support for a `DefaultLabel`. Previously, the `SelectListItem.Value` was set to `null`. This results in .NET (or, perhaps, HTML) using the label as the value, if no other value is selected. E.g., if the `TopicListAttribute`'s `DefaultValue` is set to "Select a topic…" and no topic is selected, then the value would be set to "Select a topic…". That's not what we want. By using an empty string instead, we mitigate this issue.

Author: JeremyCaney

OnTopic.Editor.AspNetCore/Components/TopicListViewComponent.cs

@@ -76,7 +76,7 @@ public IViewComponentResult Invoke(
       \-----------------------------------------------------------------------------------------------------------------------*/
       viewModel.TopicList.Add(
         new SelectListItem {
-          Value = null,
+          Value = "",
           Text = attribute.DefaultLabel
         }
       );