powerProfiles: Avoid warnings on missing power-profiles-daemon

GDBusProxy transparently handles the remote object appearing and
vanishing. It is therefore not an error if the service is not
running at the time the proxy is initialized, so we proceed and
try to read the list of profiles which is null in that case,
resulting in (harmless but annoying) warnings.

Avoid this by only reading the initial list of profiles if the
service has a name owner.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5992

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2518>