Do not enforce not NULL on tree name field
The domain clause `(name, 'not like', '%' + escape_wildcard(separator) + '%')` implicitly enforce to have a `NOT NULL` name but user could have a fallback for the record name or the `name` field could be a `Function` field for which the searcher search in many places (like the `current_name` of `BudgetLineMixin`). So the `NOT NULL` constraint on the `name` should be left to the implementation. At least until #13584 is solved.
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