sink(ticdc): precheck before split the update event (#6403)

[ti-chi-bot]

This is an automated cherry-pick of #6403

What problem does this PR solve?

Issue Number: close #6198

What is changed and how it works?

precheck before split the update event. Make sure it will not be nil.

Check List

Tests

• Manual test see: TiCDC panicked when handling update events with enable-old-value=false #6198 (comment)

Questions

Will it cause performance regression or break compatibility?

No

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

No

Release note

Fix TiCDC panic issue when disable the old value of changefeed

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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