When serializing reports, omit redundant properties

[craigfowler]

Presently, when serializing report JSON, every property is serialized. That works ok, but it results in larger-than required JSON payloads because of redundant property declarations with empty/meaningless values.

This ticket is to change the serialization so that empty values are omitted from the JSON payload. The aim being to reduce the disk size of a report.

[craigfowler]

It looks like this is the JSON serializer option I want: https://learn.microsoft.com/en-us/dotnet/api/system.text.json.jsonserializeroptions.defaultignorecondition?view=net-10.0#system-text-json-jsonserializeroptions-defaultignorecondition

[craigfowler]

Whilst I'm at it, I'd like to shorten the names of properties if I can.