sql: fix where condition without brackets problem (#372)

[ti-chi-bot]

This is an automated cherry-pick of #372

What problem does this PR solve?

close #371

What is changed and how it works?

If both conf.Where and where has a value, we use brackets to wrap these two conditions.

Check List

Tests

• Unit test
• Integration test
  Dumpling before this commit can't pass new integration tests.

Side effects

• Possible performance regression

Related changes

• Need to cherry-pick to the release branch

Release note

• fix the problem that dumpling may dump extra data if --where is specified

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• glorv

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[lichunzhu]

/merge

[ti-chi-bot]

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

Commit hash: 93c1572