fix(orderBy): ensure correct ordering with arrays of objects and no predicate

Closes angular#11866

Author: petebacondarwin

src/ng/filter/orderBy.js

@@ -241,7 +241,6 @@ function orderByFilter($parse) {
   }
 
   function objectToString(value) {
-    if (value === null) return 'null';
     if (typeof value.valueOf === 'function') {
       value = value.valueOf();
       if (isPrimitive(value)) return value;
@@ -255,7 +254,9 @@ function orderByFilter($parse) {
 
   function getPredicateValue(value, index) {
     var type = typeof value;
-    if (type === 'string') {
+    if (value === null) {
+      type = 'null';
+    } else if (type === 'string') {
       value = value.toLowerCase();
     } else if (type === 'object') {
       value = objectToString(value);