False positive for using nullable method with strict Contract("null -> null; !null -> !null") as a function !null -> !null

[unv-unv]

We have a method (toUpperCase) which returns null only if its argument is null. Otherwise it always returns not null. So it ensures non-null result for non-null argument and can be safely passed as lambda to be used in non-null context. But NullAway doesn't detect this situation and shows error:

@NullMarked
public class Test {
    @Contract("null -> null; !null -> !null")
    public @Nullable String toUpperCase(@Nullable String s) {
        return s == null ? null : s.toUpperCase(Locale.US);
    }

    public String[] map(String[] strings, Function<String, String> mapper) {
        String[] result = new String[strings.length];
        for (int i = 0, n = strings.length; i < n; i++) {
            result[i] = mapper.apply(strings[i]);
        }
        return result;
    }

    public String[] capitalizeWords(String[] words) {
        // BUG: [NullAway] referenced method returns @Nullable, but functional interface method java.util.function.Function.apply(T) returns @NonNull
        return map(words, StringUtil::toUpperCase);
    }
}

NullAway 0.13.1, Error Prone 2.48.0