Fix potentially-nullable test case

Change-Id: Ieb3d192d273b89666b8d00affe1279ada07dc384
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/104945
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Mike Fairhurst <mfairhurst@google.com>

tests/language_2/nnbd/static_errors/unchecked_use_of_nullable_test.dart

@@ -82,7 +82,7 @@ void typeParametersNullableBounds<IQ extends int?, BQ extends bool?, LQ extends
   list[0]; //# 51: compile-time error
   list[0] = 0; //# 52: compile-time error
   x += 1; //# 53: compile-time error
-  x ??= null; //# 54: ok
+  x ??= x; //# 54: ok
   x.round; //# 55: compile-time error
   x.toString; //# 56: ok
   x.noSuchMethod; //# 57: ok