Fix missing nullability on Single.subscribe(BiConsumer) (#7331)

Author: ZacSweers

src/main/java/io/reactivex/rxjava3/core/Single.java

@@ -4735,7 +4735,7 @@ public final Disposable subscribe() {
     @CheckReturnValue
     @NonNull
     @SchedulerSupport(SchedulerSupport.NONE)
-    public final Disposable subscribe(@NonNull BiConsumer<? super T, ? super Throwable> onCallback) {
+    public final Disposable subscribe(@NonNull BiConsumer<@Nullable ? super T, @Nullable ? super Throwable> onCallback) {
         Objects.requireNonNull(onCallback, "onCallback is null");
 
         BiConsumerSingleObserver<T> observer = new BiConsumerSingleObserver<>(onCallback);