[skip ci] Fix UIManagerBinding's findNodeAtPoint (facebook#42439)

Summary:

UIManagerBinding's findNodeAtPoint assumes that UIManager::findNodeAtPoint returns non-null pointers. But, this is false! See [LayoutableShadowNode::findNodeAtPoint](https://github.com/facebook/react-native/blob/fd0ca4dd6209d79ac8c93dbffac2e3dca1caeadc/packages/react-native/ReactCommon/react/renderer/core/LayoutableShadowNode.cpp#L333-L354):

https://www.internalfb.com/code/fbsource/[7169da7945c813b35cd0a4fa15e66969c90c2481]/xplat/js/react-native-github/packages/react-native/ReactCommon/react/renderer/core/LayoutableShadowNode.cpp?lines=333%2C354

Reviewed By: cortinico, sammy-SC

Differential Revision: D52909512

packages/react-native/ReactCommon/react/renderer/uimanager/UIManagerBinding.cpp

@@ -326,6 +326,12 @@ jsi::Value UIManagerBinding::get(
               arguments[3].getObject(runtime).getFunction(runtime);
           auto targetNode =
               uiManager->findNodeAtPoint(node, Point{locationX, locationY});
+
+          if (!targetNode) {
+            onSuccessFunction.call(runtime, jsi::Value::null());
+            return jsi::Value::undefined();
+          }
+
           auto& eventTarget = targetNode->getEventEmitter()->eventTarget_;
 
           EventEmitter::DispatchMutex().lock();