[Dev] Dev PR workflow fails to add link to GH issue if the body of PR is empty

[raulcd]

Describe the bug, including details regarding any error messages, version, and platform.

It has been brought up on Zulip that if the body of the PR is empty the Dev PR workflow fails and the link to the issue.

Examples:

• GH-33980: [Docs][Python] Document DataFrame Interchange Protocol implementation and usage #35835 should be linked to [Docs][Python] Document DataFrame Interchange Protocol implementation and usage #33980
• GH-35599: [Python] Canonical fixed-shape tensor extension array/type is not picklable. #35933 should be linked to [Python] Canonical fixed-shape tensor extension array/type is not picklable. #35599

Component(s)

Developer Tools

[raulcd]

This was brought up again on Zulip the error:

Run actions/github-script@v6
  with:
    github-token: ***
    script: const script = require(`${process.env.GITHUB_WORKSPACE}/.github/workflows/dev_pr/link.js`);
  script({github, context});

    debug: false
    user-agent: actions/github-script
    result-encoding: json
    retries: 0
    retry-exempt-status-codes: 400,401,403,404,422

TypeError: Cannot read properties of null (reading 'includes')
Error: Unhandled error: TypeError: Cannot read properties of null (reading 'includes')
    at commentGitHubURL (/home/runner/work/arrow/arrow/.github/workflows/dev_pr/link.js:87:43)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)
    at async module.exports (/home/runner/work/arrow/arrow/.github/workflows/dev_pr/link.js:107:7)

happened here: https://github.com/apache/arrow/actions/runs/5455750281/jobs/9927653433