Allows modules to not provide any type definition if they provide at least one provider

In Accounts.js 1.0 certain modules like the mongo module are in charge to solely provide a `DatabaseInterfaceUserToken` or a `DatabaseInterfaceSessionsToken`: they basically implement a known interface which other modules can use to access either the database used to store user data or the one used to store sessions.
Unfortunately graphql-modules complains about missing `typeDefs` so I had to work it around by passing [fake ones](https://github.com/accounts-js/accounts/blob/master/modules/module-mongo/src/index.ts#L35).
Another use case could be modules used solely to provide services consumed by REST-only users.
In either case we shouldn't be forced to provide `typeDefs` in each and every module if we want to allow the user to use modules to piece together their application.