ReactRootView is Nullable in ReactDelegate (#46440)

Summary:
Pull Request resolved: #46440

ReactDelegate does not properly annotate the ReactRootView field as Nullable. This fixes that.

## Changelog

[Android][Fixed] Use appropriate Nullable attribute for ReactRootView field in ReactDelegate

Reviewed By: philIip

Differential Revision: D62514424

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

@@ -29,7 +29,7 @@
 public class ReactDelegate {
 
   private final Activity mActivity;
-  private ReactRootView mReactRootView;
+  @Nullable private ReactRootView mReactRootView;
 
   @Nullable private final String mMainComponentName;
 
@@ -305,6 +305,7 @@ public void loadApp(String appKey) {
     }
   }
 
+  @Nullable
   public ReactRootView getReactRootView() {
     if (ReactFeatureFlags.enableBridgelessArchitecture) {
       return (ReactRootView) mReactSurface.getView();