[BUG] Avoid logging error when mimir is not set

At the moment , see a lot error like the following in thornode log

docker-thornode-1  | 4:59AM ERR fail to fetch mimir value error=null key=KillSwitchStart
docker-thornode-1  | 4:59AM ERR fail to fetch mimir value error=null key=KillSwitchDuration
docker-thornode-1  | 4:59AM INF handleMsgSwitch request destination address=tthor1p97szszykk5g5chfqe7sras4du775uvj68z2sh
docker-thornode-1  | 4:59AM ERR fail to fetch mimir value error=null key=KillSwitchStart
docker-thornode-1  | 4:59AM ERR fail to fetch mimir value error=null key=KillSwitchDuration

These are actually not error , so it should be silent, no error