[RadioGroup] Support defaultValue in uncontrolled mode (#14092)

* Respect defaultValue for uncontrolled RadioGroup

* Remove props from RadioGroup super() and split uncontrolled/defaultValue tests

* Change RadioGroup isControlled check to compare with null

packages/material-ui/src/RadioGroup/RadioGroup.js

@@ -9,13 +9,15 @@ import { createChainedFunction, find } from '../utils/helpers';
 class RadioGroup extends React.Component {
   radios = [];
 
-  state = {
-    value: null,
-  };
-
   constructor(props) {
     super();
     this.isControlled = props.value != null;
+
+    if (!this.isControlled) {
+      this.state = {
+        value: props.defaultValue,
+      };
+    }
   }
 
   focus = () => {

packages/material-ui/src/RadioGroup/RadioGroup.test.js

@@ -56,6 +56,35 @@ describe('<RadioGroup />', () => {
     );
   });
 
+  it('should support default value in uncontrolled mode', () => {
+    const wrapper = shallow(
+      <RadioGroup name="group" defaultValue="zero">
+        <Radio value="zero" />
+        <Radio value="one" />
+      </RadioGroup>,
+    );
+
+    assert.strictEqual(
+      wrapper
+        .children()
+        .first()
+        .props().checked,
+      true,
+    );
+
+    const radio = wrapper.children().last();
+    const event = { target: { value: 'one' } };
+    radio.simulate('change', event, true);
+
+    assert.strictEqual(
+      wrapper
+        .children()
+        .last()
+        .props().checked,
+      true,
+    );
+  });
+
   describe('imperative focus()', () => {
     let wrapper;