[bugfix] Don't error when populating MovedTo if account not found

[tsmethurst]

Description

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If this is a documentation change, please briefly describe what you've changed and why.

We may not always have the MovedTo account stored in the database when an account's MovedToURI is set. It's nice if we do have it, since we can then convey that via the API too, but it's fine if we don't, and we don't need to error for it.

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 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.