cockpithandlers: Be defensive if PrettyHostname is not set

The JS code will crash if the variables aren't defined; let's emit the
empty string in this case.

The real bug was that I had SELinux enforcing but with no policy for
Cockpit, so cockpitd couldn't talk to systemd-hostnamed.  But let's
still submit this defensive patch.

Closes #149

Signed-off-by: Stef Walter <stefw@redhat.com>