Avoid setting the optional UID argument for the external authentication.
It fixes using gdbus from a user namespace where geteuid() can be different than the uid in the namespace where the server is running.
Based on https://github.com/systemd/systemd/commit/1ed4723d38cd0d1423c8fe650f90fa86007ddf55
Signed-off-by: Giuseppe Scrivano gscrivan@redhat.com