This was previously part of the accounts management, but that has since been removed.

Not prefixing with self makes subcategory a local variable, meaning that the method has no effect

Mostly fix name pluralization and slight logic tweaks

This is not a very necessary refactor, it was fine as is. This is mostly done to mirror the same pattern in another model.

Removing the nil checks will not give errors when sorting valid objects. If it is invalid or being constructed, that may be an issue.

Even if objects are invalid (e.g. if an InvoiceLine has an invalid category), allow them to still be sorted without raising errors.

Self is needed in the null_subcategory helper to prevent it creating a local variable. It is not needed elsewhere.

This is not a validation step; rather, it runs before validation. Either of blank or nil continues to be allowed.