Modify Namespace spec factory to always have a type

What does this MR do and why?

Modifies namespace factory to explicitly create User type namespaces.

Previously, nil namespaces meant personal (user) namespaces. We've moved away from that, backfilling the User namespace models to have type User rather than nil. This is part of an effort to prohibit nil type namespaces generally and make them all explicitly typed.

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Related to #341447 (closed)