checkpercona-replication-delay: explicitly check if $serverid is undef otherwise valid variables will still fail

Signed-off-by: Christophe Vanlancker <christophe.vanlancker@inuits.eu>
checkpercona-replication-delay: shorter syntax to check undef condition

Signed-off-by: Christophe Vanlancker <christophe.vanlancker@inuits.eu>