Serialize empty conversation account list as empty list, not null

[VyrCossont]

Description

This pull request fixes a type error when serializing a conversation's account list. This is only about the millionth time I've run into this problem; golang/go#63397 when?

Followup to #3013

Impact

Only affects DMs to yourself and clients that are strict about types. Semaphore and Phanpy apparently don't care.

Checklist

• 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 not leveraged AI to create the proposed changes.
• 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.

[tsmethurst]

Ah thanks for fixing this, I just ran into it myself :)