Allow components to render undefined (#21869)

Author: rickhanlonii

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

@@ -924,33 +924,37 @@ describe('ReactDOMServerIntegration', () => {
       );
     });
 
-    describe('components that throw errors', function() {
-      itThrowsWhenRendering(
-        'a function returning undefined',
-        async render => {
-          const UndefinedComponent = () => undefined;
-          await render(<UndefinedComponent />, 1);
-        },
-        'UndefinedComponent(...): Nothing was returned from render. ' +
-          'This usually means a return statement is missing. Or, to ' +
-          'render nothing, return null.',
-      );
+    describe('components that render nullish', function() {
+      itRenders('a function returning null', async render => {
+        const NullComponent = () => null;
+        await render(<NullComponent />);
+      });
 
-      itThrowsWhenRendering(
-        'a class returning undefined',
-        async render => {
-          class UndefinedComponent extends React.Component {
-            render() {
-              return undefined;
-            }
+      itRenders('a class returning null', async render => {
+        class NullComponent extends React.Component {
+          render() {
+            return null;
           }
-          await render(<UndefinedComponent />, 1);
-        },
-        'UndefinedComponent(...): Nothing was returned from render. ' +
-          'This usually means a return statement is missing. Or, to ' +
-          'render nothing, return null.',
-      );
+        }
+        await render(<NullComponent />);
+      });
+
+      itRenders('a function returning undefined', async render => {
+        const UndefinedComponent = () => undefined;
+        await render(<UndefinedComponent />);
+      });
+
+      itRenders('a class returning undefined', async render => {
+        class UndefinedComponent extends React.Component {
+          render() {
+            return undefined;
+          }
+        }
+        await render(<UndefinedComponent />);
+      });
+    });
 
+    describe('components that throw errors', function() {
       itThrowsWhenRendering(
         'a function returning an object',
         async render => {