Handle unset env var checks safely

[Phlogistique]

Motivation

• Prevent the script from failing under set -u when checking an unset environment variable name.
• Ensure check_env_var performs a safe indirect expansion to avoid Bash errors for missing variables.

Description

• Updated the check_env_var function in update-pr-stack.sh to use a safe expansion: [[ -z "${!1-}" ]] instead of [[ -z "${!1}" ]].
• This avoids an unbound variable error when evaluating the value of a possibly-undefined environment variable name.

Testing

• No automated tests were executed as part of this change.
• The repository contains test scripts under tests/ but they were not run for this patch.

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Codex Task