When a category's download path option is set to "Default", its `downloadPath` is serialized into JSON as `undefined`. This results in the `downloadPath` field being omitted from `torrents/categories` and `torrents/maindata` payloads (as is expected with an `undefined` value).
The use of `undefined` here causes an issue in the WebUI. Specifically, when the category previously contained a value for this field (i.e. download path option set to either "Yes" or "No"), the `processMap` logic in `SyncController` does not detect the removal this field. This results in the category's new `downloadPath` not being properly sent to the client. By switching from `undefined` to `null`, we ensure that the `downloadPath` value is always included in the category's payload. This allows `processMap` to properly detect whenever the value changes.
This change is backwards compatible with existing categories.json files. Older qBittorrent versions should also be able to parse new categories.json files containing `null`.
More context: cd3fbfbf9b (r2173148696)
PR #22958.
2ebe8595c9