This ticket covers the leftover scenarios from subissue 1 from ZBX-27736

1) discard preprocessing
nodata() must fire (not become unknown) when entries are discarded by pre-processing,

2) log metadata
nodata() must fire (not become unknown) when only metadata log entries are received, without actual value

3) old entries
nodata() must fire (not become unknown) when only old entries from the past (outside nodata period) are received

Other point should already be handled in ZBX-27736
1) unsupported item
nodata() must fire (not become unknown) when history for unsupported items is received (e.g. int item receives string history)
2) nodata suppressed when there is connection loss between Zabbix server and Zabbix proxy

check that there are no unknown triggers/events and most important no problems after connection has been restored. but also nodata can fire before value processed

Removing unknowns is in ZBXNEXT-10636