Export GLOBAL_NAMESERVERS_YAML consistently across all return paths

[Copilot]

This stacked PR addresses review feedback on the custom nameservers implementation in container-entrypoint.sh.
The issue was inconsistent export behaviour: GLOBAL_NAMESERVERS_YAML was exported only in the non-empty path, but not in early-return empty paths.

• What changed

  • Updated build_global_nameservers_yaml() to export GLOBAL_NAMESERVERS_YAML in both empty-list branches:
    • when ns_array is empty
    • when all parsed entries are invalid and items is empty
  • Kept existing parsing, validation, and YAML generation logic unchanged.

• Behavioural impact

  • GLOBAL_NAMESERVERS_YAML is now exported uniformly regardless of whether nameservers are present, absent, or filtered out.
  • This removes path-dependent environment behaviour and avoids surprises if the function is reused independently.

• Code snippet

  if [[ ${#ns_array[@]} -eq 0 ]]; then
    export GLOBAL_NAMESERVERS_YAML='[]'
    return
  fi
  
  if [[ ${#items[@]} -eq 0 ]]; then
    export GLOBAL_NAMESERVERS_YAML='[]'
    return
  fi

[github-actions]

🆕 New Headscale Config Options

+ - 2606```

[github-actions]

🆕 New Headscale Config Options

+ - 2606```