Add check for string and null 'rootElement' in ReactDOMFiber (#9869)

* Add check for string and null 'rootElement' in ReactDOMFiber

**what is the change?:**
Before we call 'rootElement.getAttribute' we check that the method is
defined.

**why make this change?:**
There is an internal use case I found where 'rootElement' is a string
and null at different points as the page is rendered.

It looks like this method was added as part of support for re-hydration
of server-side rendered content. I can't imagine we would want to reuse
content if the rootnode is a string or null. Not sure if we want an
earlier check that it's an element before this point.

**test plan:**
`yarn test`
and I manually tested this fix in the internal case where it was
breaking

* Add test and improve check for non-element rootElement

**what is the change?:**
We use the nodeType to check that we have the correct type of
rootElement, and we added a unit test.

**why make this change?:**
Improve this solution to the problem.

**test plan:**
`yarn test`

* run ./scripts/fiber/record-tests

scripts/fiber/tests-passing.txt

@@ -1314,6 +1314,7 @@ src/renderers/dom/shared/__tests__/ReactRenderDocument-test.js
 * should not be able to unmount component from document node
 * should not be able to switch root constructors
 * should be able to mount into document
+* renders over an existing text child without throwing
 * should give helpful errors on state desync
 * should throw on full document render w/ no markup
 * supports findDOMNode on full-page components

src/renderers/dom/fiber/ReactDOMFiber.js

@@ -116,7 +116,9 @@ function getReactRootElementInContainer(container: any) {
 
 function shouldReuseContent(container) {
   const rootElement = getReactRootElementInContainer(container);
-  return !!(rootElement && rootElement.getAttribute(ID_ATTRIBUTE_NAME));
+  return !!(rootElement &&
+    rootElement.nodeType === ELEMENT_NODE &&
+    rootElement.getAttribute(ID_ATTRIBUTE_NAME));
 }
 
 function shouldAutoFocusHostComponent(type: string, props: Props): boolean {

src/renderers/dom/shared/__tests__/ReactRenderDocument-test.js

@@ -178,6 +178,13 @@ describe('rendering React components at document', () => {
     expect(testDocument.body.innerHTML).toBe('Hello world');
   });
 
+  it('renders over an existing text child without throwing', () => {
+    const container = document.createElement('div');
+    container.textContent = 'potato';
+    ReactDOM.render(<div>parsnip</div>, container);
+    expect(container.textContent).toBe('parsnip');
+  });
+
   it('should give helpful errors on state desync', () => {
     class Component extends React.Component {
       render() {