fix: avoid unbound variable errors for NO_CHANGELOG_LABEL and BASE_REF

[Ocheretovich]

Rationale: With set -u, if NO_CHANGELOG_LABEL=true and BASE_REF is not set, the script was failing before the early return could exit successfully. Moving the BASE_REF check below the early return fixes this.

[huitseeker]

BASE_REF is now required before checking NO_CHANGELOG_LABEL, so a run with
NO_CHANGELOG_LABEL=true and no BASE_REF will fail instead of taking the intended successful early exit for “no changelog” PRs.

[github-actions]

Automated check (CONTRIBUTING.md)

Findings:

• Link an issue in the PR body (e.g., "Fixes Implement SHA256 in Miden Assembly #123").
• Change appears to be a trivial code-only edit without tests; may be batched internally.

Recommendations:

• Consider adding a Test plan or clear review steps.

Next steps:

• Link a relevant issue (e.g., "Fixes Implement SHA256 in Miden Assembly #123") and ensure it is assigned to you.
• See CONTRIBUTING.md for expectations.
• If this is a false positive, comment: /quality-review.