The code currently assumes that if the fingerprint is non-null it's
valid, but gnome-remote-desktop can also set the fingerprint to an
empty string if there's no certificate yet.
This commit makes sure to handle both NULL and empty fingerprints.
If the username and password get set then the certificate fingerprint
isn't updated even if that lets the service start.
This commit fixes things by doing a round trip through the service after
setting the username/password to
1. Make sure the change actually worked
2. Trigger our fingerprint dialog setting code that currently only runs
on reading username/password
