If a config server has the following setting document:

{ "_id" : "balancer", "_secondaryThrottle" : false, "mode" : "off" }

sh.getBalancerState() will return true because there is no "stopped" field, despite the mode field being set to "off."

This incorrect reporting can cause confusion. We should error when a field we expect to exist does not exist. If it's expected that the stopped field may not exist, then we shouldn't rely on it for checking the balancer state.

Please be mindful of internal callers of sh.getBalancerState() like sh.status() when implementing this change.