align shallow renderer with other renderers in defaulting state to null on mount (#11965)

Author: jwbay

src/ReactShallowRenderer.js

@@ -125,7 +125,7 @@ class ReactShallowRenderer {
   _mountClassComponent(props, context) {
     this._instance.context = context;
     this._instance.props = props;
-    this._instance.state = this._instance.state || emptyObject;
+    this._instance.state = this._instance.state || null;
     this._instance.updater = this._updater;
 
     if (typeof this._instance.componentWillMount === 'function') {

src/__tests__/ReactShallowRenderer-test.js

@@ -947,4 +947,17 @@ describe('ReactShallowRenderer', () => {
     renderAndVerifyWarningAndError([], 'array');
     renderAndVerifyWarningAndError({}, 'object');
   });
+
+  it('should have initial state of null if not defined', () => {
+    class SomeComponent extends React.Component {
+      render() {
+        return <span />;
+      }
+    }
+
+    const shallowRenderer = createRenderer();
+    shallowRenderer.render(<SomeComponent />);
+
+    expect(shallowRenderer.getMountedInstance().state).toBeNull();
+  });
 });