pkg/infoschema: fix slow_query TestSlowQuery attrs expectation

Author: jiong-nba

pkg/infoschema/test/clustertablestest/tables_test.go

@@ -495,6 +495,7 @@ func TestSlowQuery(t *testing.T) {
 			"60e9378c746d9a2be1c791047e008967cf252eb6de9167ad3aa6098fa2d523f4",
 			"",
 			"update t set i = 2;",
+			"null",
 			"select * from t_slim;",
 		},
 		{"2021-09-08 14:39:54.506967",
@@ -588,6 +589,7 @@ func TestSlowQuery(t *testing.T) {
 			"",
 			"",
 			"",
+			"null",
 			"INSERT INTO ...;",
 		},
 	}