[TestUtils.act] fix return result checking (#14758)

* fix .act return value testing when result === null

* nit

Author: Sunil Pai

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

@@ -665,6 +665,12 @@ describe('ReactTestUtils', () => {
   });
 
   it('warns if you return a value inside act', () => {
+    expect(() => act(() => null)).toWarnDev(
+      [
+        'The callback passed to ReactTestUtils.act(...) function must not return anything.',
+      ],
+      {withoutStack: true},
+    );
     expect(() => act(() => 123)).toWarnDev(
       [
         'The callback passed to ReactTestUtils.act(...) function must not return anything.',

packages/react-dom/src/test-utils/ReactTestUtils.js

@@ -397,7 +397,7 @@ const ReactTestUtils = {
     if (__DEV__) {
       if (result !== undefined) {
         let addendum;
-        if (typeof result.then === 'function') {
+        if (result !== null && typeof result.then === 'function') {
           addendum =
             '\n\nIt looks like you wrote ReactTestUtils.act(async () => ...), ' +
             'or returned a Promise from the callback passed to it. ' +

packages/react-noop-renderer/src/createReactNoop.js

@@ -878,7 +878,7 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
       if (__DEV__) {
         if (result !== undefined) {
           let addendum;
-          if (typeof result.then === 'function') {
+          if (result !== null && typeof result.then === 'function') {
             addendum =
               "\n\nIt looks like you wrote ReactNoop.act(async () => ...) or returned a Promise from it's callback. " +
               'Putting asynchronous logic inside ReactNoop.act(...) is not supported.\n';

packages/react-test-renderer/src/ReactTestRenderer.js

@@ -571,7 +571,7 @@ const ReactTestRendererFiber = {
     if (__DEV__) {
       if (result !== undefined) {
         let addendum;
-        if (typeof result.then === 'function') {
+        if (result !== null && typeof result.then === 'function') {
           addendum =
             "\n\nIt looks like you wrote TestRenderer.act(async () => ...) or returned a Promise from it's callback. " +
             'Putting asynchronous logic inside TestRenderer.act(...) is not supported.\n';