fix: rootKey empty check by len equals 0 (backport #801)

[mergify]

This is an automatic backport of pull request #801 done by Mergify.
Cherry-pick of 06f5be1 has failed:

On branch mergify/bp/release/v0.21.x/pr-801
Your branch is up to date with 'origin/release/v0.21.x'.

You are currently cherry-picking commit 06f5be1.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md
	both modified:   iterator.go
	both modified:   iterator_test.go

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[julienrbrt]

Not applicable.