selftest: txtimestamp: fix net ns entry logic

According to 'man 8 ip-netns', if `ip netns identify` returns an empty string,
there's no net namespace associated with current PID: fix the net ns entrance
logic.

Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>