Make fields property optional on SimpleMessagesProvider

[TiBianMod]

this PR makes the fields property optional on SimpleMessagesProvider

on docs, I see that the fields property is not required on SimpleMessagesProvider

Now if you want to use SimpleMessagesProvider you need to pass a second argument

new SimpleMessagesProvider({
  'required': 'The {{ field }} field is required',
}, {})

[TiBianMod]

cc, @thetutlage @RomainLanz

[thetutlage]

Thanks 👍