Fix loading of legacy registry.json files

[HebaruSan]

Problem

If you install some mods with the current release of CKAN and then switch to the current dev build, it will show no mods installed.

Thanks to @JonnyOThan for reporting this on Discord.

Cause

In #3904, we retired Registry.available_modules and Registry.download_counts. However, since the old client will crash if these are absent or null, we created a JsonAlwaysEmptyObjectConverter to ensure they're always saved as an empty JSON object ({}). This converter's deserialization function worked by simply creating an empty copy of the required type (Dictionary<string, string> in this case).

However, a JsonConverter's ReadJson implementation must call JToken.Load(reader) to extract the current token from the input stream! We weren't doing that, and the parse was stopping right after that point, before Registry.installed_modules was deserialized. Maybe Newtonsoft.Json considers that an error condition?

This doesn't happen with a "new" registry because it doesn't call the converter when the JSON object is already empty (I think).

Changes

Now JsonAlwaysEmptyObjectConverter.ReadJson calls JToken.Load(reader) and ignores its return value before it returns the empty dictionary, which allows the full Registry object to finish deserializing.

I'll probably self-review this because it's small and a regression.