make t_server_null 'server alive?' check more robust

make t_server_null 'server alive?' check more robust

- use "$RUN_SUDO kill -0 $pid" to test if a given process is running, not
  "ps -p $pid" - the latter will not work if security.bsd.see_other_uids=0
  is set

- produce proper error messages if pid files can not be found or are
  empty at server shutdown time

Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20240918162917.6809-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg29314.html

Signed-off-by: Gert Doering <gert@greenie.muc.de>