Fixed a bug where id's wouldn't be generated on empty strings.

src/Core/Extensions/EnumerableExtensions.cs

@@ -9,7 +9,7 @@ public static void EnsureIds<T>(this IEnumerable<T> values, Func<T, string> gene
                 return;
 
             foreach (var value in values) {
-                if (value.Id == null)
+                if (String.IsNullOrEmpty(value.Id))
                     value.Id = generateIdFunc(value);
             }
         }