Inconsistent value for missing instance ports in process stats

GET /v3/processes/:guid/stats returns inconsistent values for non-existing instance_ports:

• missing instance_ports.internal and instance_ports.external are reported as 0
• missing instance_ports.internal_tls_proxy_port and instance_ports.external_tls_proxy_port are reported as null

capi-release v1.210.0 changed the values for a missing instance_ports.internal and instance_ports.external to null as a side effect of updating protobufs (#4359).

This (unintended) change broke cf-java-client because it checks for non-null instance_ports.internal and instance_ports.external.

#4401 fixes/changes missing instance_ports values back to the in-consistent values described above. Released with capi-release v1.211.0.

This issue is to find a long-term solution that returns consistent values for missing ports.

Proposal is to consolidate on null eventually. Can be done only when cf-java-client allows null values, cf-java-client #1284.

Start with the GET /v3/processes/:guid/stats endpoint and the changes described in #4401, then review cf-java-client #1284 for the compatibility dependency. Done means the project has an agreed consistent representation for missing instance ports and the affected client behavior is accounted for.