Small cleanup before PR

Author: marc-antoine-girard

Editor/Items/NoneDropdownItem.cs

@@ -11,7 +11,7 @@ public NoneDropdownItem() : base("None") { }
 
         public object GetValue()
         {
-            return default;
+            return null;
         }
     }
-}
+}

Editor/Utilities/SerializableInterfaceAdvancedDropdown.cs

@@ -61,6 +61,7 @@ protected override AdvancedDropdownItem BuildRoot()
                         (Comparison<AdvancedDropdownItem>)Sort, true
                     });
             }
+
             return item;
         }