Gets the credentials of the authenticated peer.
This will always return null unless this acted as a server (e.g. g_dbus_connection_flags_authentication_server was passed) when set up and the client passed credentials as part of the authentication process.
In a message bus setup, the message bus is always the server and each application is a client. So this method will always return null for message bus clients.
a Credentials or null if not available. Do not free this object, it is owned by this.