[code-simplifier] refactor: use getErrorMessage helper in extra_empty_commit.cjs (#23155 follow-up)

[github-actions]

This PR simplifies actions/setup/js/extra_empty_commit.cjs — recently modified in #23155 — to use the existing getErrorMessage utility from error_helpers.cjs instead of duplicating the inline error extraction pattern.

Files Simplified

• actions/setup/js/extra_empty_commit.cjs — replaced two inline error instanceof Error ? error.message : String(error) expressions with getErrorMessage(error) and added the corresponding import

Improvements Made

Applied project standard error-handling pattern:

• Added const { getErrorMessage } = require("./error_helpers.cjs"); import
• Replaced error instanceof Error ? error.message : String(error) (×2) with getErrorMessage(error)

This is already the established pattern across 15+ other files in actions/setup/js/ (e.g., add_comment.cjs, add_labels.cjs, validate_secrets.cjs). extra_empty_commit.cjs missed this helper because it was added before the helper was introduced (or was added independently), and PR #23155 introduced two new instances of the inline pattern.

Note: error_helpers.cjs's getErrorMessage is slightly more thorough than the inline ternary — it also handles objects with a message property that aren't Error instances — so this is a minor behavior improvement as well.

Changes Based On

• 🛠️ Improve error messaging for branch sync failure #23155 — 🛠️ Improve error messaging for branch sync failure

Testing

• ✅ extra_empty_commit.test.cjs — all 35 tests pass
• ✅ Linting passes (make lint-cjs)
• ✅ Formatting passes (make fmt-cjs)
• ✅ No functional changes — error message output is identical for standard Error objects

Review Focus

Please verify:

• The import and two replacement usages are correct
• Behavior is preserved (it is: getErrorMessage returns error.message for Error instances, same as the inline ternary)

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Pull request overview

This PR refactors actions/setup/js/extra_empty_commit.cjs to use the shared getErrorMessage helper for consistent error-message extraction, removing duplicated inline logic.

Changes:

• Import getErrorMessage from ./error_helpers.cjs.
• Replace two occurrences of error instanceof Error ? error.message : String(error) with getErrorMessage(error).

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