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

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[ti-chi-bot]

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