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.
Suppresses the following warning:
qBittorrent/src/base/bittorrent/categoryoptions.cpp: In static member function ‘static BitTorrent::CategoryOptions BitTorrent::CategoryOptions::fromJSON(const QJsonObject&)’:
qBittorrent/src/base/bittorrent/categoryoptions.cpp:44:59: warning: missing initializer for member ‘BitTorrent::CategoryOptions::DownloadPathOption::path’ [-Wmissing-field-initializers]
44 | options.downloadPath = {downloadPathValue.toBool()};
Change "Incomplete/temp folder" term with "download folder".
Allow to set "download folder" per torrent (in manual mode) and per category (in automatic mode).
