Bug 742200. Use JSVAL_VOID, not JSVAL_NULL, as the default value for optional arguments that are not specified. r=peterv

dom/bindings/Codegen.py

@@ -1391,7 +1391,7 @@ class CGArgumentConverter(CGThing):
             "index" : index,
             "argc" : argc,
             "argv" : argv,
-            "defaultValue" : "JSVAL_NULL",
+            "defaultValue" : "JSVAL_VOID",
             "name" : "arg%d" % index
             }
         if argument.optional: