Fixes children when using dangerouslySetInnerHtml in a selected <option> (#13078)

* Fixes children when using dangerouslySetInnerHtml in a selected <option>

This fixes an inadvertent cast of undefined children to an empty string when creating an option tag that will be selected:

```
  <select defaultValue="test">
    <option value='test' dangerouslySetInnerHTML={{ __html: '&rlm; test'}} />
  </select>
```

This causes an invariant error because both children and dangerouslySetInnerHTML are set.

* PR fix and new ReactDOMServerIntegrationForms test

* Account for null case

* Combine test cases into single test

* Add tests for failure cases

* Fix lint

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

@@ -443,6 +443,37 @@ describe('ReactDOMSelect', () => {
     expect(markup).not.toContain('<option selected="" value="gorilla"');
   });
 
+  it('should support server-side rendering with dangerouslySetInnerHTML', () => {
+    const stub = (
+      <select defaultValue="giraffe">
+        <option
+          value="monkey"
+          dangerouslySetInnerHTML={{
+            __html: 'A monkey!',
+          }}>
+          {undefined}
+        </option>
+        <option
+          value="giraffe"
+          dangerouslySetInnerHTML={{
+            __html: 'A giraffe!',
+          }}>
+          {null}
+        </option>
+        <option
+          value="gorilla"
+          dangerouslySetInnerHTML={{
+            __html: 'A gorilla!',
+          }}
+        />
+      </select>
+    );
+    const markup = ReactDOMServer.renderToString(stub);
+    expect(markup).toContain('<option selected="" value="giraffe"');
+    expect(markup).not.toContain('<option selected="" value="monkey"');
+    expect(markup).not.toContain('<option selected="" value="gorilla"');
+  });
+
   it('should support server-side rendering with multiple', () => {
     const stub = (
       <select multiple={true} value={['giraffe', 'gorilla']} onChange={noop}>

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

@@ -35,6 +35,7 @@ const {
   resetModules,
   itRenders,
   itClientRenders,
+  itThrowsWhenRendering,
   renderIntoDom,
   serverRender,
 } = ReactDOMServerIntegrationUtils(initModules);
@@ -327,6 +328,81 @@ describe('ReactDOMServerIntegration', () => {
         expectSelectValue(e, 'bar');
       });
 
+      itRenders(
+        'a select with options that use dangerouslySetInnerHTML',
+        async render => {
+          const e = await render(
+            <select defaultValue="baz" value="bar" readOnly={true}>
+              <option
+                id="foo"
+                value="foo"
+                dangerouslySetInnerHTML={{
+                  __html: 'Foo',
+                }}>
+                {undefined}
+              </option>
+              <option
+                id="bar"
+                value="bar"
+                dangerouslySetInnerHTML={{
+                  __html: 'Bar',
+                }}>
+                {null}
+              </option>
+              <option
+                id="baz"
+                value="baz"
+                dangerouslySetInnerHTML={{
+                  __html: 'Baz',
+                }}
+              />
+            </select>,
+            1,
+          );
+          expectSelectValue(e, 'bar');
+        },
+      );
+
+      itThrowsWhenRendering(
+        'a select with option that uses dangerouslySetInnerHTML and 0 as child',
+        async render => {
+          await render(
+            <select defaultValue="baz" value="foo" readOnly={true}>
+              <option
+                id="foo"
+                value="foo"
+                dangerouslySetInnerHTML={{
+                  __html: 'Foo',
+                }}>
+                {0}
+              </option>
+            </select>,
+            1,
+          );
+        },
+        'Can only set one of `children` or `props.dangerouslySetInnerHTML`.',
+      );
+
+      itThrowsWhenRendering(
+        'a select with option that uses dangerouslySetInnerHTML and empty string as child',
+        async render => {
+          await render(
+            <select defaultValue="baz" value="foo" readOnly={true}>
+              <option
+                id="foo"
+                value="foo"
+                dangerouslySetInnerHTML={{
+                  __html: 'Foo',
+                }}>
+                {''}
+              </option>
+            </select>,
+            1,
+          );
+        },
+        'Can only set one of `children` or `props.dangerouslySetInnerHTML`.',
+      );
+
       itRenders(
         'a select value overriding defaultValue no matter the prop order',
         async render => {

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

@@ -258,7 +258,10 @@ function flattenTopLevelChildren(children: mixed): FlatReactChildren {
   return [fragmentChildElement];
 }
 
-function flattenOptionChildren(children: mixed): string {
+function flattenOptionChildren(children: mixed): ?string {
+  if (children === undefined || children === null) {
+    return children;
+  }
   let content = '';
   // Flatten children and warn if they aren't strings or numbers;
   // invalid types are ignored.