Take advantage of delete not acting on null values

Nebula.h and User.h
Fix 'one start' with existent DB but nonexistent auth file

The start script assumes that an existing database implies
credentials located in either one of these places:
  * $HOME/.one/one_auth or
  * $ONE_AUTH

Now, if the database was setup once correctly but the
credentials file does not exist or ONE_AUTH is not set
anymore, oned runs into a segmentation fault.

The change prevents this situation by always checking for
user credentials file wheter the DB exists or not.