Backport of Audit: comment updates and conditional merge into release/1.17.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #27963 to be assessed for backporting due to the inclusion of the label backport/1.17.x.

The below text is copied from the body of the original PR.

---

Description

• updates comments to inform callers about the nil, nil potential return value.
• merges two consecutive if conditions in clientCertSerialNumber func

HashiCorp Checklist

• Labels: If this PR is the CE portion of an ENT change, and that ENT change is
  getting backported to N-2, use the new style backport/ent/x.x.x+ent labels
  instead of the old style backport/x.x.x labels.
• Labels: If this PR is a CE only change, it can only be backported to N, so use
  the normal backport/x.x.x label (there should be only 1).
• ENT Breakage: If this PR either 1) removes a public function OR 2) changes the signature
  of a public function, even if that change is in a CE file, double check that
  applying the patch for this PR to the ENT repo and running tests doesn't
  break any tests. Sometimes ENT only tests rely on public functions in CE
  files.
• Jira: If this change has an associated Jira, it's referenced either
  in the PR description, commit message, or branch name.
• RFC: If this change has an associated RFC, please link it in the description.
• ENT PR: If this change has an associated ENT PR, please link it in the
  description. Also, make sure the changelog is in this PR, not in your ENT PR.

---

Overview of commits

• d824e57

[github-actions]

CI Results:
All required Go tests succeeded but failures were detected ⚠️
Failures:

Test Type	Package	Test	Logs
race	builtin/logical/pkiext	Test_ZLintECDSA256	view test results
race	builtin/logical/pkiext	Test_ZLintECDSA384	view test results
race	builtin/logical/pkiext	Test_ZLintECDSA521	view test results
race	builtin/logical/pkiext	Test_ZLintRSA2048	view test results
race	builtin/logical/pkiext	Test_ZLintRSA2048PSS	view test results
race	builtin/logical/pkiext	Test_ZLintRSA3072	view test results
race	builtin/logical/pkiext	Test_ZLintRSA3072PSS	view test results

[github-actions]

Build Results:
All builds succeeded! ✅