Werk #2756: SNMP: Generalized error handling of emtpy info for SNMP checks

This change changes the way empty data for SNMP checks is handled. In previous version each SNMP check had to deal with the fact that the SNMP agent could respond with an empty response (info = []).

This situation can better be handled in the Check_MK SNMP engine which catches this issue now before calling the discovery/check function of the check. The error is then reported as missing info like for the Check_MK Agent based checks.

There might be some special SNMP checks which need to treat this case of empty data on its own. These checks now need to declare the attribute handle_empty_info in the check_info.

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