Skip notification on unstable health with monitoring_samples > 1

When using monitoring_samples > 1 with the Consul hostmonitor driver, mixed health samples (e.g., [‘up’, ‘up’, ‘down’]) result in an unstable health state containing None values. The _event() method incorrectly treated this as a stable ‘up’ state because ‘down’ not in [None] evaluates to True, causing false STARTED notifications.

This change adds an explicit check for None in host_health to skip notifications when the health state is unstable. This prevents premature STARTED events during host transition from up to down.