Add safeguard to downloader: Do not offer 0 byte files for download

As the Elevate_Winter.zip theme file download is currently broken (0 byte file on server), a follow-up problem surfaced: The changed file still appeared as an update (which is technically correct due to newer file timestamp), so updating that file erases the existing one with a broken one. c:geo downloaders should not offer 0 byte files as updates.