Avoid panic in cronjob sorting

This change handles the case where the ith cronjob may have its start
time set to nil.

Previously, the Less method could cause a panic in case the ith
cronjob had its start time set to nil, but the jth cronjob did not. It
would panic when calling Before on a nil StartTime.

pkg/controller/cronjob/utils.go

@@ -196,13 +196,14 @@ func (o byJobStartTime) Len() int      { return len(o) }
 func (o byJobStartTime) Swap(i, j int) { o[i], o[j] = o[j], o[i] }
 
 func (o byJobStartTime) Less(i, j int) bool {
-	if o[j].Status.StartTime == nil {
-		return o[i].Status.StartTime != nil
+	if o[i].Status.StartTime == nil && o[j].Status.StartTime != nil {
+		return false
+	}
+	if o[i].Status.StartTime != nil && o[j].Status.StartTime == nil {
+		return true
 	}
-
 	if o[i].Status.StartTime.Equal(o[j].Status.StartTime) {
 		return o[i].Name < o[j].Name
 	}
-
 	return o[i].Status.StartTime.Before(o[j].Status.StartTime)
 }