Removed String() call in favor of variable substitution. (#9829)

[ti-chi-bot]

This is an automated cherry-pick of #9829

What problem does this PR solve?

Issue Number: close #9676

What is changed and how it works?

Substitute String() call for variable comparison:
emptyMySQLGTIDSet, _ = gmysql.ParseMysqlGTIDSet("")
emptyMariaDBGTIDSet, _ = gmysql.ParseMariadbGTIDSet("")
to speed up algorithm.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

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

Please refer to [Release Notes Language Style Guide](https://pingcap.github.io/tidb-dev-guide/contribute-to-tidb/release-notes-style-guide.html) to write a quality release note.

If you don't think this PR needs a release note then fill it with `None`.

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
jenkins-ticdc/verify	2edc394	link	true	/test verify
pull-dm-integration-test	2edc394	link	true	/test dm-integration-test
pull-dm-compatibility-test	2edc394	link	true	/test dm-compatibility-test

Full PR test history. Your PR dashboard.

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