Add presence validation to platform and token to be consistent with the database

[rmehner]

The database schema marks these two columns as non-nullable, so I think it might be nice to also reflect that in the validations on the model itself.

We stumbled upon this when using database_consistency to check our models and database to be consistent.

[rosa]

Hey @rmehner, thanks for this! This was done on purpose, however, following the same convention as other Rails gems, where DB constraints aren't necessarily translated into model validations.

Thanks a lot in any case, and sorry for the delay!