ci: guard updates for missing submodules

[nuclearcat]

What changed

Guard each external-submodule update so it only runs when the checked-out
submodule has its own .git marker.

Why

issue_comment workflows are loaded from the default branch, then the job
checks out the pull request head. A pull request that removes a submodule can
therefore run the older default-branch workflow against a tree where that
submodule no longer exists.

This caused staging deployment run 29489094118 to fail while trying to enter
kernelci.org/external/kernelci-core; kcidb would have failed next.

Impact

Existing submodules continue to update from their main branches. Removed or
uninitialized submodules are skipped, allowing submodule-removal pull requests
to use the staging deployment workflow safely.

Validation

• Parsed .github/workflows/deploy.yml with PyYAML.
• Ran git diff --check.
• Verified missing or uninitialized submodule paths are skipped successfully.

[patersonc]

Hi @nuclearcat

Thanks for this. Do you want to add your SOB to the commit?

[patersonc]

/staging

[github-actions]

Your PR has been deployed to staging. Staging URL: https://static.staging.kernelci.org/ After testing, you can merge your PR. Note: This is a temporary staging URL, if anyone else will test another PR, it will override contents.

[nuclearcat]

I think it will refuse to touch/test workflows changes, i will merge as is and test another PR.