RolePermissions is defined in the XML schema as a sequence with minOccurs="0", which means this is optional.
When missing, the namespace default defined in the <Model> tag should be used.

Technically, missing means, that the element does not exist at all.

If RolePermissions exists as an empty Tag <RolePermissions />, this means the node has no role permissions.
This can be intended to hide some nodes. This should not lead to loading the namespace default.

This interpretation makes sense IMO, but it should be written to the spec to avoid different ways of interpretation.