The problem I can see here is if the string None is used in other contexts. Nenhuma is a “female pronoun” that fits the portuguese word notification (also female), but if this word is male then the correct form would be Nenhum. The word Nada doesn’t have this issue, although it can feel a bit more awkward in certain situations.
So, I guess you should use Nenhuma if you are fairly sure this string will only be used along with the notification schedule feature.