First proc.cpu.util[] request results in network error

When agent receives new proc.cpu.util[] request it adds new query to collector and returns OK with no value. While this 'hack' works with simple items, it is interpreted as network failure by server when processing agent checks.

One solution would be to wait for collector to gather at least 2 samples (or timeout) before returning a real value.

proc.cpu.util[] was implemented in ZBXNEXT-494. This issue was noticed in sub-issue (9).