replaced null check with `hasContentDimensions` (#69164)

packages/flutter/lib/src/widgets/nested_scroll_view.dart

@@ -802,7 +802,7 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
 
   bool get hasScrolledBody {
     for (final _NestedScrollPosition position in _innerPositions) {
-      assert(position.minScrollExtent != null && position.hasPixels);
+      assert(position.hasContentDimensions && position.hasPixels);
       if (position.pixels > position.minScrollExtent) {
         return true;
       }