bus-proxyd: ignore errors from sd_bus_creds_get_well_known_names()

sd_bus_creds_get_well_known_names() fails with -ENODATA in case the
message has no names attached, which is intended behavior if the
remote connection didn't own any names at the time of sending.

The function already deals with 'sender_names' being an empty strv,
so we can just continue in such cases.