Fix null crash when using maintainVisibleContentPosition on Android (#38891)

Summary:
`mFirstVisibleView` is a weak ref so it can also be null when dereferencing.

This was reported on the original PR here https://github.com/facebook/react-native/pull/35049#discussion_r1288195469

## Changelog:

[ANDROID] [FIXED] - Fix null crash when using maintainVisibleContentPosition on Android

Pull Request resolved: https://github.com/facebook/react-native/pull/38891

Test Plan: Not sure exactly in what cases this can happen, but the fix is trivial and makes sense.

Reviewed By: cortinico

Differential Revision: D48192154

Pulled By: rshest

fbshipit-source-id: 57a38a22a0e216a33603438355bde0013c014fbf

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/views/scroll/MaintainVisibleScrollPositionHelper.java

@@ -103,6 +103,10 @@ public class MaintainVisibleScrollPositionHelper<ScrollViewT extends ViewGroup &
     }
 
     View firstVisibleView = mFirstVisibleView.get();
+    if (firstVisibleView == null) {
+      return;
+    }
+
     Rect newFrame = new Rect();
     firstVisibleView.getHitRect(newFrame);