* do not suppress NoData state
* extract conversion of state to postable alert + tests
* create a special alert instance if nodata
* use NoData when converting from Keep Last State instead of Alerting
* add silence during migration if NoData is mapped to KeepLastState.
(cherry picked from commit 610643a668)
Co-authored-by: Yuriy Tseretyan <yuriy.tseretyan@grafana.com>