querier.Select cannot return a nil series set.

Signed-off-by: gotjosh <josue.abreu@gmail.com>

rules/group.go

@@ -675,18 +675,18 @@ func (g *Group) RestoreForState(ts time.Time) {
 			continue
 		}
 
-		// No results for this alert rule.
-		if sset == nil {
-			level.Debug(g.logger).Log("msg", "Failed to find a series to restore the 'for' state", labels.AlertName, alertRule.Name())
-			continue
-		}
-
 		// While not technically the same number of series we expect, it's as good of an approximation as any.
 		seriesByLabels := make(map[string]storage.Series, alertRule.ActiveAlertsCount())
 		for sset.Next() {
 			seriesByLabels[sset.At().Labels().DropMetricName().String()] = sset.At()
 		}
 
+		// No results for this alert rule.
+		if len(seriesByLabels) == 0 {
+			level.Debug(g.logger).Log("msg", "Failed to find a series to restore the 'for' state", labels.AlertName, alertRule.Name())
+			continue
+		}
+
 		alertRule.ForEachActiveAlert(func(a *Alert) {
 			var s storage.Series