Builder: return null if no spec supplied

https://fedorahosted.org/freeipa/ticket/3235

install/ui/src/freeipa/_base/Builder.js

@@ -25,6 +25,8 @@ define(['dojo/_base/declare',
         './Spec_mod'
         ], function(declare, array, lang, construct, Spec_mod) {
 
+    var undefined;
+
     var Builder = declare(null, {
         /**
          * Builds objects based on specication.
@@ -81,6 +83,8 @@ define(['dojo/_base/declare',
 
             var f,c;
 
+            if (spec === undefined || spec === null) return null;
+
             context = context || {};
 
             if (overrides) {