server: emit empty fields when rendering protos as JSON
server: emit empty fields when rendering protos as JSON

Fixes #6927.

glycerine/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,glycerine/cockroach,sunya123/cockroach,vivekmenezes/cockroach,petermattis/cockroach,glycerine/cockroach,veteranlu/cockroach,glycerine/cockroach,maxlang/cockroach,vaibhavarora/cockroach,maxlang/cockroach,veteranlu/cockroach,veteranlu/cockroach,paperstreet/cockroach,veteranlu/cockroach,sunya123/cockroach,arjun4084346/cockroach,sunya123/cockroach,maxlang/cockroach,paperstreet/cockroach,petermattis/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,vivekmenezes/cockroach,petermattis/cockroach,maxlang/cockroach,veteranlu/cockroach,maxlang/cockroach,vivekmenezes/cockroach,vivekmenezes/cockroach,veteranlu/cockroach,sunya123/cockroach,vaibhavarora/cockroach,petermattis/cockroach,paperstreet/cockroach,arjun4084346/cockroach,paperstreet/cockroach,vivekmenezes/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,paperstreet/cockroach,glycerine/cockroach,arjun4084346/cockroach,sunya123/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,cockroachdb/cockroach,arjun4084346/cockroach,paperstreet/cockroach,sunya123/cockroach,vivekmenezes/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,petermattis/cockroach,maxlang/cockroach,sunya123/cockroach,petermattis/cockroach,arjun4084346/cockroach,maxlang/cockroach,cockroachdb/cockroach,vivekmenezes/cockroach,maxlang/cockroach,arjun4084346/cockroach,glycerine/cockroach,arjun4084346/cockroach,cockroachdb/cockroach,glycerine/cockroach,glycerine/cockroach,petermattis/cockroach,arjun4084346/cockroach,sunya123/cockroach,petermattis/cockroach,paperstreet/cockroach,cockroachdb/cockroach