Actually skip prereleases

Since b7fa87cb9b21aceaf0cb0f9b612dde4f91ca6655 only stable releases are supposed to be considered. Instead however, these were still added, but with an empty hash/url/version which breaks with hard-to-trace eval errors when thrown against `fetchNextcloudApp`[1] Out-of-bounds list index at «none»:0: (source not available) [1] https://github.com/NixOS/nixpkgs/issues/239011