Hide TopicListViewComponent if no values are returned

There will always be at least one value—for the `DefaultLabel`—so if the `TopicList.Count` isn't greater than `1` then hide the attribute from the interface entirely. This includes hiding the `Layout`, as otherwise the attribute wrapper will be displayed without the dropdown list.

Author: JeremyCaney

OnTopic.Editor.AspNetCore/Areas/Editor/Views/Editor/Components/TopicList/Default.cshtml

@@ -1,14 +1,18 @@
 @model TopicListAttributeViewModel
 
 @{
+  if (Model.TopicList.Count > 1) {
     Layout = "~/Areas/Editor/Views/Editor/Components/_Layout.cshtml";
+  }
 }
 
-<select
-  asp-for                       ="Value"
-  asp-items                     ="Model.TopicList"
-  class                         ="@Model.AttributeDescriptor.CssClass form-control form-inline"
-  disabled                      =@(!Model.AttributeDescriptor.IsEnabled)
-  required                      =@Model.AttributeDescriptor.IsRequired
-  >
-</select>
+@if (Model.TopicList.Count > 1) {
+  <select
+    asp-for                       ="Value"
+    asp-items                     ="Model.TopicList"
+    class                         ="@Model.AttributeDescriptor.CssClass form-control form-inline"
+    disabled                      =@(!Model.AttributeDescriptor.IsEnabled)
+    required                      =@Model.AttributeDescriptor.IsRequired
+    >
+  </select>
+}