fix: do not render mocked Modal when visible=false (#39157)

Summary:
Note: this PR is related only to testing mocks provided by RN, and does not affect runtime code.

When building new Jest matchers for React Native Testing Library (callstack/react-native-testing-library#1468) I've noticed that when rendering React Native in Jest using React Test Renderer the mocked `Modal` component renders host `Modal` element when `visible={false}`.

This seems to be incorrect as only visible modal should render any host elements. Not visible one should not render any host element even empty ones. Current mock implementation also contradicts the behaviour of non-mocked `Modal` which does not render `RCTModalHostView` in such case.

## Changelog:

[General] [Fixed] - Do not render mocked <Modal /> when `visible=false`

Pull Request resolved: #39157

Test Plan:
I've added test showing that non-mocked Modal renders to `null` and modifies the existing tests so that mocked Modal also renders to `null.`

Luna: I've updated relevant snapshots

Reviewed By: NickGerleman

Differential Revision: D48728277

Pulled By: lunaleaps

fbshipit-source-id: cf06495ad959e2d9549241b57f46f75d7beb9eae

packages/react-native/Libraries/Modal/__tests__/Modal-test.js

@@ -26,13 +26,13 @@ describe('<Modal />', () => {
     expect(instance).toMatchSnapshot();
   });
 
-  it('should not render its children when mocked with visible=false', () => {
+  it('should not render <Modal> when mocked with visible=false', () => {
     const instance = render.create(
       <Modal visible={false}>
         <View testID="child" />
       </Modal>,
     );
-    expect(instance.root.findAllByProps({testID: 'child'})).toHaveLength(0);
+    expect(instance.toJSON()).toBeNull();
   });
 
   it('should shallow render as <Modal> when mocked', () => {
@@ -65,4 +65,15 @@ describe('<Modal />', () => {
     );
     expect(instance).toMatchSnapshot();
   });
+
+  it('should not render <RCTModalHostView> when not mocked with visible=false', () => {
+    jest.dontMock('../Modal');
+
+    const instance = render.create(
+      <Modal visible={false}>
+        <View />
+      </Modal>,
+    );
+    expect(instance.toJSON()).toBeNull();
+  });
 });

packages/react-native/Libraries/Modal/__tests__/__snapshots__/Modal-test.js.snap

@@ -13,7 +13,7 @@ exports[`<Modal /> should render as <RCTModalHostView> when not mocked 1`] = `
 <RCTModalHostView
   animationType="none"
   hardwareAccelerated={false}
-  identifier={4}
+  identifier={3}
   onDismiss={[Function]}
   onStartShouldSetResponder={[Function]}
   presentationStyle="fullScreen"

packages/react-native/jest/mockModal.js

@@ -17,11 +17,13 @@ import typeof Modal from '../Libraries/Modal/Modal';
 
 function mockModal(BaseComponent: $FlowFixMe) {
   class ModalMock extends BaseComponent {
-    render(): React.Element<Modal> {
+    render(): React.Element<Modal> | null {
+      if (this.props.visible === false) {
+        return null;
+      }
+
       return (
-        <BaseComponent {...this.props}>
-          {this.props.visible !== true ? null : this.props.children}
-        </BaseComponent>
+        <BaseComponent {...this.props}>{this.props.children}</BaseComponent>
       );
     }
   }