Don't send a reply if NO_REPLY_EXPECTED is set. This is already taken
care of by g_dbus_method_invocation_return_gerror(), so we only need
to do this for the non-error case, since we generate the reply message
and manually send it.
Because we have to keep arguments alive for the duration of the call,
and this requires us to pass a ready callback even in case of
NO_REPLY_EXPECTED, there is a challenge: If the implementation is a
GDBus proxy, the presence of a ready callback means we don't set
NO_REPLY_EXPECTED, even if we don't want the reply. But since we know
that a proxy will copy the arguments right away, we can omit the ready
callback in that case.
