title: Zabbix trigger to detect no data
body: <p>I need trigger able to detect that polled Zabbix agent items does not returns data.
  For zabbix trapper items this functionality is covered by nodata() function (Heartbeat lost detection in Zabbix documentation) but I need similar functionality supported for Zabbix agent items.</p>
  
  <p>For example, a have defined Zabbix agent UserParameter:
  <code>UserParameter=custom.mssqlping,/usr/local/scripts/mssqlping.sh</code>.</p>
  
  <p>The script mssqlping.sh returns 0/1. I need to cover situation when mssqlping.sh script is broken and returns empy string, which is not stored in zabbix because it is not number.</p>
  
  <p>How to detect that periodically polled item data are no longer coming?</p>