syncer: save empty SQL when online DDL is filtered (#1659)

[ti-chi-bot]

This is an automated cherry-pick of #1659

What problem does this PR solve?

close #1656

What is changed and how it works?

• save an empty SQL when online DDL is filtered, and remove it when read it from online DDL component
• use ? not fmt.Sprintf to build a query

Check List

Tests

• Integration test

Code changes

• Has persistent data change (not likely, because if there's quote in source-id, an error already occurred)

Side effects

• Possible performance regression
• Increased code complexity

Related changes

• Need to cherry-pick to the release branch
• Need to be included in the release note

[lichunzhu]

/run-compatibility-test

[lichunzhu]

/run-all-tests

[lichunzhu]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lichunzhu

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[lichunzhu]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 7f028bf