Fix textarea bug

The test changes revealed a bug with textarea. It happens because we
currently always insert trailing comment nodes. We should optimize that
away. However, we also don't really support complex children so we
should toString it anyway which is what partial renderer used to do.

packages/react-dom/src/__tests__/ReactDOMServerIntegrationTextarea-test.js

@@ -49,6 +49,12 @@ describe('ReactDOMServerIntegrationTextarea', () => {
     expect(e.value).toBe('foo');
   });
 
+  itRenders('a textarea with a value of undefined', async render => {
+    const e = await render(<textarea value={undefined} />);
+    expect(e.getAttribute('value')).toBe(null);
+    expect(e.value).toBe('');
+  });
+
   itRenders('a textarea with a value and readOnly', async render => {
     const e = await render(<textarea value="foo" readOnly={true} />);
     // textarea DOM elements don't have a value **attribute**, the text is

packages/react-dom/src/server/ReactDOMServerFormatConfig.js

@@ -1001,7 +1001,17 @@ function pushStartTextArea(
     target.push(leadingNewline);
   }
 
-  return value;
+  // ToString and push directly instead of recurse over children.
+  // We don't really support complex children in the value anyway.
+  // This also currently avoids a trailing comment node which breaks textarea.
+  if (value !== null) {
+    if (__DEV__) {
+      checkAttributeStringCoercion(value, 'value');
+    }
+    target.push(stringToChunk(encodeHTMLTextNode('' + value)));
+  }
+
+  return null;
 }
 
 function pushSelfClosing(