@Nonnull
    public static<T> NullableOptional<T> empty() {
        @SuppressWarnings("unchecked")
        NullableOptional<T> t = (NullableOptional<T>) EMPTY;
        return t;
    }
Returns an empty {@code NullableOptional} instance. No value is present for this
NullableOptional.

@param <T> Type of the non-existent value
@return an empty {@code NullableOptional}
helper/src/main/java/me/lucko/helper/utils/NullableOptional.java