fix: fix invalid trigger logic

This should check if file is already generated or it or if there is just
no server configured. These are two separate conditions when there is no
need to do migration. The first case is when migration is done and
second case is when there just nothing to migrate.

This should have minor impact. It only removes error:
  fix-dhparam-to-cagen/postinst: switch: CA 'openvpn' doesn't exist
This attempted to migration when there was no configuration. This is not
case when migration is required but not performed and thus there should
be no real issue we need to address.

updater/fix/files/dhparam-to-cagen

@@ -3,7 +3,7 @@ set -e
 
 dhparam="/etc/ssl/ca/openvpn/dhparam.pem"
 
-if [ -f "$dhparam" ] && ! uci get openvpn.server_turris >/dev/null 2>&1; then
+if [ -f "$dhparam" ] || ! uci get openvpn.server_turris >/dev/null 2>&1; then
 	exit 0
 fi