xkcd: address py/mixed-returns CodeQL alert

[dgw]

Description

I started this branch intending to fixup several other alerts, but realized that most of them were of the same type we already decided to dismiss (no one really cares about the return value of a plugin callable).

However, xkcd has a real function that was flagged by this rule, so I chose to still fix it even though it means opening a dinky +1 pull request. But I won't bother milestoning this until it's merged, so it doesn't affect our release timelines in any way.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make lint and make test)
• I have tested the functionality of the things this change touches