Fix formatting of user in RestApiServlet if username is not set

The error message from RestApiServlet that is displayed if a user
doesn't have a required capability, prints the user's username. This
username may be null, as fallback now the fullname and preferred email
address of the user are printed.

Change-Id: I786be9d1772b76eb6712d49cebe867b20d32d975
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>