Ensure that searchHits is not null in AlertConditions

Under some conditions FieldValueAlertCondition#getSearchHits and MessageCountAlertCondition#getSearchHits
could return null which caused problems further downstream, when consumers of the respective AlertCondition
expected to always get a list of messages (though being empty if there weren't any search hits).

Refs Graylog2/graylog2-alarmcallback-slack#1