ddl_sink (ticdc): fix ddl sink nil point panic (#6390)

[ti-chi-bot]

This is an automated cherry-pick of #6390

What problem does this PR solve?

Issue Number: close #4934

What is changed and how it works?

Check List

Tests

• Integration test
• Manual test (add detailed scripts or steps below)

1. Add a time.Sleep(time.Second * 10) above: https://github.com/pingcap/tiflow/blob/master/cdc/owner/ddl_sink.go#L111
2. make cdc
3. start cdc server
4. create a changefeed with syncpoint enable
5. no panic
6. pause changefeed and update it, disable syncpoint , rusume this changefeed
7. no panic

Questions

Will it cause performance regression or break compatibility?

Do you need to update user documentation, design documentation or monitoring documentation?

Release note

Fix ddl sink panic when changefeed syncpoint is enable.

[ti-chi-bot]

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[ti-chi-bot]

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