A null there indicates a deserialisation error and therefore due to the
catch block immediately succeeding the changed line everything will
continue to work as intended.
As reported in #12718, it turns out that temporary files from beatmap
set downloads performed via the beatmap listing overlay could remain in
the user's filesystem even after the download has concluded.
The reason for the issue is a failure in component integration.
In the case of online downloads, files are first downloaded to a
temporary directory (`C:/Temp` or `/tmp`), with a randomly generated
filename, which ends in an extension of `.tmp`.
On the other side, `ArchiveModelManager`s have a `ShouldDeleteArchive()`
method, which determines whether a file should be deleted after
importing. At the time of writing, in the case of beatmap imports the
file is only automatically cleaned up if the extension of the file is
equal to `.osz`, which was not the case for temporary files.
As it turns out, `APIDownloadRequest` has a facility for adjusting the
file's extension, via the protected `FileExtension` property. Therefore,
use it in the case of `DownloadBeatmapSetRequest` to specify `.osz`,
which then will make sure that the `ShouldDeleteArchive()` check in
`BeatmapManager` picks it up for clean-up.
Until now, API requests sent to dummy API were just lost in the void. In most cases this somehow worked as expected, but any logic which is waiting on a request to finish will potentially never get a response.
Going forward, I'm not 100% sure that every `Wait` on a web response will have local timeout logic (I think there is a certain amount of assumption that this is being managed for us by `APIAccess`), so I've made this change to better handle such cases going forward. Now, rather than nothing happening, requests will trigger a failure via the existing exception logic rather than silently pretending the request never arrived.
