Defined ZBX_SERVER and ZBX_SERVER_PORT but empty

If ZBX_SERVER and ZBX_SERVER_PORT variables are defined, but empty in zabbix.inc.php, Zabbix frontend uses incorrect configuration:
1. ":10051" if ZBX_SERVER is defined but empty
2. "hostname" if ZBX_SERVER_PORT is defined but empty
If ZBX_SERVER is empty - Zabbix frontend must use DB data to determine correct Zabbix server location.