Steps to reproduce:

1. Create item as system.sw.os from default template or any other item of type character with some large update interval
2. Create trigger like: last(/Zabbix server/system.sw.os,#1)<>last(/Zabbix server/system.sw.os,#2) and length(last(/Zabbix server/system.sw.os))>0
   1. This is how it is defined in the default template for trigger "Operating system description has changed"

Result:
This trigger will fire after collecting first value

Expected:
It must not fire as we have no second value to evaluate last(#2)