Update invalid render error message

forgot to update the message following the previous diff on rendering null.

src/core/ReactCompositeComponent.js

@@ -1181,7 +1181,7 @@ var ReactCompositeComponentMixin = {
       invariant(
         ReactDescriptor.isValidDescriptor(renderedComponent),
         '%s.render(): A valid ReactComponent must be returned. You may have ' +
-          'returned an array or some other invalid object.',
+          'returned undefined, an array or some other invalid object.',
         this.constructor.displayName || 'ReactCompositeComponent'
       );
       return renderedComponent;

src/core/__tests__/ReactCompositeComponent-test.js

@@ -159,8 +159,8 @@ describe('ReactCompositeComponent', function() {
       ReactTestUtils.renderIntoDocument(<Component />);
     }).toThrow(
       'Invariant Violation: Component.render(): A valid ReactComponent must ' +
-      'be returned. You may have returned an array or some other invalid ' +
-      'object.'
+      'be returned. You may have returned undefined, an array or some other ' +
+      'invalid object.'
     );
   });