I think it's very reasonable to add a 'hasAccounts' boolean to the MailAPI.  Arguably it's pretty fundamental to the mail library to know if there are any accounts or not.  It's entirely reasonable for us to defer the announcing of the full Mail API until we at least know that information (even if the accounts have not been loaded yet.)

The main reasons we require a call to viewAccounts() or viewFolders() to create a slice to get that info is that:
- those instances exist 1:1 with each call to viewAccounts()/viewFolders() so that 'data' and 'element' fields can be set on the instances without conflicting with other UI bindings displaying the same data.
- there is a cost to sending updates on the accounts so we want a UI/our  message channel to not get those if it doesn't want them.