Partitioned downstreams with peered upstreams could not properly merge central config info (i.e. proxy-defaults and service-defaults things like mesh gateway modes) if the upstream had an empty DestinationPartition field in Enterprise.
Due to data flow, if this setup is done using Consul client agents the field is never empty and thus does not experience the bug.
When a service is registered directly to the catalog as is the case for consul-dataplane use this field may be empty and and the internal machinery of the merging function doesn't handle this well.
This PR ensures the internal machinery of that function is referentially self-consistent.
R.B. Boyer