[chore] tidy + test timelines a bit better

[tsmethurst]

Description

I'm trying to narrow down a bug where an account with an empty timeline trying to do a GET to their timeline locks up the timeline manager for everyone on the instance. Haven't found it yet, but did these little improvements and extra tests while I was thinking.

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.