ReactRootView.createRootView should be @nullable (#45924)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/45924

Changelog: [android][fixed] Anntate ReactRootView.createRootView with nullable

**Context**

- D54772205 changed `createRootView` to return `null`
- https://www.internalfb.com/diff/D60803470?dst_version_fbid=533309559035246&transaction_fbid=475433162004889

**Change**

- Annotate `createRootView` with `Nullable`

Reviewed By: mdvacca

Differential Revision: D60867547

fbshipit-source-id: 12c325cb9a2dd6db4b2388b2ad3c2af13065d371

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/ReactActivityDelegate.java

@@ -70,7 +70,7 @@ public class ReactActivityDelegate {
    *
    * <p>Not used on bridgeless
    */
-  protected ReactRootView createRootView() {
+  protected @Nullable ReactRootView createRootView() {
     return null;
   }