Inconsistent responses between read-children-types and read-children-names or read-children-resources

The read-children-types op is implemented by reading the names of the NodeSubregistry objects in the target resource's ManagementResourceRegistration. The read-children-names and read-children-resources ops are implemented by reading the full PathElement of registered child MRRs.

It's possible these two can get out of sync, as a NodeSubregistry can be added in order to support a submodel, but such subregistries aren't removed if all children are removed. This manifests itself in EAP 6 with the 'server' child type. See https://bugzilla.redhat.com/show_bug.cgi?id=1093030. (Note WF should not have this specific problem, as WF registers a generic MRR for 'server'.)

Fix is for GlobalOperationHandlers.getChildAddresses() to include an entry with an empty value for any valid child type. This will ensure the responses are consistent.