libregraph: allow provisioning with empty displayName

We should allow users without a displayname and distinguish between surname and givenname to present more personalized messages.

We don't invent a username and persist that, instead when the user is authenticated and we mint a token we fill the displayname property of the user object on the fly. Furthermore, we need to fall back to the username when listing shares and share recipients.

The optimal solution would check if the displayname is set and fall back to username or email.