scheduler(ticdc): fix incorrect scheduling task counter (#9840)

[ti-chi-bot]

This is an automated cherry-pick of #9840

What problem does this PR solve?

Issue Number: close #9845

What is changed and how it works?

1. Do not generate remove task if len(tables)==0

Check List

Tests

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

Questions

Will it cause performance regression or break compatibility?

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

Release note

`None`

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This pull-request has been approved by: 3AceShowHand, nongfushanquan

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