fix(dependency_updater): guard against index out of range when branchCommit is empty

[ygd58]

In dependency_updater.go getVersionAndCommit(), branchCommit[0].SHA is accessed without checking if the slice is empty. If the GitHub API returns zero commits for the branch, this panics with index out of range. Added a len check that returns a descriptive error instead.

[github-actions]

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