storage: fix s3 storage walk-dir with empty sub-dir (#713)

[ti-srebot]

cherry-pick #713 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In br repo:
git pr https://github.com/pingcap/br/pull/733

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/br.git pr/733:release-4.0-605037ce1716

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What problem does this PR solve?

Close #712

What is changed and how it works?

The s3 implement ListObjects will return no data if set the prefix to / instead of leave it empty.

NOTE: the current implementation is compatible with minio but no s3.

Check List

Tests

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

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

Release Note

• Fix the bug that WalkDir for s3 storage returns nil if the target path is bucket name.

[3pointer]

/lgtm

[kennytm]

/lgtm