example: beatmapsets/1329817#osu/2778259
Basically how those empty diff works is that they're using a really long audio file (maybe a blank one to preserve filesize to the minimum) and put few objects at the end of the diff to bypass the limit and extend the filesize allowed to be uploaded.osu! wiki wrote:
Beatmaps will fail to submit if they exceed the online file size or difficulty limit. The file size limit is 5MB plus an additional 10MB for every minute of beatmap length, and it caps at 100MB.