forced chown of /var/log/zabbix should not be in debian postinst

Steps to reproduce:
Upgrade of zabbix server on a system that is using syslog for logging.

Result:
No logs are written anymore since the syslog server no longer has write access to the log files.

Expected:
No change to the ownership if the directory is not empty (not first install). The post install script should assume that the directory ownership is correct for the system configuration on upgrade.