Don't restart openvpn if it's not installed

src/freedombone-app-vpn

                VPN_TLS_PORT)
 
 function logging_on_vpn {
+    if [ ! -f /etc/openvpn/server.conf ]; then
+        return
+    fi
     sed -i 's|status .*|status /var/log/openvpn.log|g' /etc/openvpn/server.conf
     systemctl restart openvpn
 }
 
 function logging_off_vpn {
+    if [ ! -f /etc/openvpn/server.conf ]; then
+        return
+    fi
     sed -i 's|status .*|status /dev/null|g' /etc/openvpn/server.conf
     systemctl restart openvpn
 }