Handle a user whose account was deleted.

[tvalentyn]

One of PR Bot workflows is failing because the PR #29347 has a comment from a now-deleted account.

Attempting to fix this in this PR.

https://github.com/apache/beam/actions/runs/7602366264/job/20702841060

Processing PR 29347
/runner/_work/beam/beam/scripts/ci/pr-bot/lib/updateReviewers.js:93
            if (comment.user.login &&
                             ^

TypeError: Cannot read properties of null (reading 'login')
    at /runner/_work/beam/beam/scripts/ci/pr-bot/lib/updateReviewers.js:93:30
    at Generator.next (<anonymous>)
    at fulfilled (/runner/_work/beam/beam/scripts/ci/pr-bot/lib/updateReviewers.js:22:58)
    at runMicrotasks (<anonymous>)
    at processTicksAndRejections (node:internal/process/task_queues:96:5)
Error: Process completed with exit code 1.

[tvalentyn]

R: @Abacn

[tvalentyn]

Test (in flight): https://github.com/apache/beam/actions/runs/7750207100

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[Abacn]

CC: @shunping

[Abacn]

Thanks, LGTM