[UMBRELLA] use "omitempty" in json REST object definition

None of the json definitions (DAO) use the option omitempty. Adding this allows removing on default values (like: 0, false, nil slices, maps and pointers)

There is an impact on the code: the webapp needs to be able to handle missing entries and default them correctly.

It also requires documentation updates as we currently can show empty values in the output.

Third part is to extend this all the way into the k8shim cache that creates the cache dumps.