"Spec" nightly workflow occasionally fails due to `TypeError: Cannot read properties of undefined (reading 'map')`

The nightly "Spec" workflow failed last night with the following traceback:

> matrix-spec@0.0.1 get-proposals
> node ./scripts/proposals.js

/home/runner/work/matrix-spec/matrix-spec/scripts/proposals.js:201
    const labelsForIssue = issue.labels.map(l => l.name);
                                        ^

TypeError: Cannot read properties of undefined (reading 'map')
    at /home/runner/work/matrix-spec/matrix-spec/scripts/proposals.js:201:41
    at Array.filter (<anonymous>)
    at processIssues (/home/runner/work/matrix-spec/matrix-spec/scripts/proposals.js:200:36)
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)

Node.js v20.19.3

This is the script in question:

https://github.com/matrix-org/matrix-spec/blob/fb4a0d8f66fb6263786ab7c4de3ef21517fcc64c/scripts/proposals.js#L3-L13

which is used to build the Spec Change Proposals page.

This appears to be only an occasional failure, one that has happened before. Re-running the script manually today yielded success.

From the traceback and the occasional success, my guess is that the GitHub API is occasionally returning the wrong thing. The only impact of this is the spec proposals page getting out of date by a day or two, so fairly low-priority.

Start in scripts/proposals.js at processIssues around lines 200-201, then inspect the nightly workflow in .github/workflows/main.yml and the linked failed run. Run the proposals script to observe the intermittent behavior. Done means the nightly workflow no longer fails when the API response lacks the expected labels and the proposals page can still be generated.