fix(alerting): make sure xorm can reset execution_error

pkg/services/sqlstore/alert.go

@@ -222,7 +222,11 @@ func SetAlertState(cmd *m.SetAlertStateCommand) error {
 		alert.State = cmd.State
 		alert.StateChanges += 1
 		alert.NewStateDate = time.Now()
-		alert.ExecutionError = cmd.Error
+		if cmd.Error == "" {
+			alert.ExecutionError = " " //without this space, xorm skips updating this field
+		} else {
+			alert.ExecutionError = cmd.Error
+		}
 
 		sess.Id(alert.Id).Update(&alert)
 		return nil