The registry.json file must be loaded and parsed before CKAN can do anything (CLI, GUI, etc.). Currently this file is about 21 MB and takes about 2 seconds to load on an SSD. Nearly 6 MB of it just sets assorted properties to null. But guess what value the deserializer uses if the property is absent? Yep: null. So those properties don't need to be there.

This pull request marks all the JsonProperty attributes that have at least one null value in registry.json with NullValueHandling = NullValueHandling.Ignore, which means that if the value is null, it will not be printed to the file. When the properties are not null, they will be saved and loaded normally. The file will load the same and CKAN will work the same either way.

$ ls -gG

-rw-r--r-- 1 21217174 Nov  9 16:58 before.json
-rw-r--r-- 1 15360478 Nov  9 17:22 debloat.json

Total change: 5856696 bytes, or 5.86 MB, or 27.6% of the current file size. This may yield a noticeable performance improvement on systems where registry.json is stored on a slower disk. (I don't think it's a straight 25% speed-up because the deserializer still needs to create the same objects and so on.)

There are still "ksp_version":null properties in my registry for a few mods with no version requirements. I think this is due to some custom deserialization code adjusting things independently of the attributes. It seems harmless for now.

Inspired by code review of #1888.