Caught panic when canceling DDL jobs

[lance6716]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

check the error message of

tidb/pkg/executor/recover_test.go

Lines 578 to 582 in 2088b34

	// test PD connection issue causes failure after tidb_ddl_error_count_limit
	testfailpoint.Enable(t, "github.com/pingcap/tidb/pkg/ddl/mockClearTablePlacementAndBundlesErr", `return()`)
	// TODO(lance6716): fix it later
	//tk.MustGetErrMsg("flashback database test_flashback", "[ddl:-1]DDL job rollback, error msg: mock error for clearTablePlacementAndBundles")
	tk.MustExecToErr("flashback database test_flashback")

2. What did you expect to see? (Required)

no panic message

3. What did you see instead (Required)

[ddl:-1]panic in handling DDL logic and error count beyond the limitation 2, cancelled

4. What is your TiDB version? (Required)

[lance6716]

The reason is when a job can't be converted to rollingback job in convertJob2RollbackJob, the returned error is nil. And nil error will cause needUpdateRawArgs returns true. Then the RawArgs will be marshalled from Args but Args is nil and not initialized. Then the new job will panic when unmarshal from empty RawArgs.