...G...... [ZBX-8664] allow empty values for agent items


git-svn-id: svn://svn.zabbix.com/trunk@48579 f4e7fd02-d210-4e34-a1e0-d4093fb66cc2
.......... [ZBX-8664] add missing ChangeLog entry


git-svn-id: svn://svn.zabbix.com/trunk@48580 f4e7fd02-d210-4e34-a1e0-d4093fb66cc2