[chore] small changes missed in previous dereferencer.GetAccount() PRs

[NyaaaWhatsUpDoc]

• 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.
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• 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]

Perhaps before returning, it's worth calling processing.LoadAttachment to make sure that the attachment will really be in the database for the caller to select with the ID; it's idempotent anyway I think so it should be fine.