Return false when change_position is not in correct format

What does this MR do and why?

On production, there are some old DiffNote records with change_position set to JSON string in the DB so rails can't serialize it back to Gitlab::Diff::Position.

This results to error on discussions endpoint when loading a MR.

undefined method `on_file?' for an instance of String (NoMethodError)

    return false if change_position&.on_file?

This seems to be legacy data error. To fix the endpoint to not error out, we return false early when change_position isn't in the correct format we expect.

References

gitlab-com/gl-infra/production#22194

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