Enforce personal namespaces to not be allowed as subgroups

While working on !44740 (merged) an affected spec was failing due to a wrong setup of nested namespaces. The setup was that a namespace was nested under a group. But we don't allow personal namespaces as subgroups. So we should evaluate if it makes sense to add a validation and/or a spec to enforce this and avoid future confusion about this (suggestion was made here).