If you're seeing a 0kb file and you don't know why, there's a pretty solid chance you're using Firefox. Firefox has been flagging Forge files as malware for every version of Forge after .108 (for 1.13.2 releases), but unlike Chrome, which tells you what's happening and makes it obvious, all Firefox does is place a little yellow dot in the download notification icon (see attached).
You have to open that menu and click on the "more information link" under the file it flagged, at which point Firefox will say OMG THIS FILE IS GOING TO DESTROY YOUR COMPUTER PROBABLY but it gives you the option to either delete the file or open it. If you choose to open the file and then click cancel on the dialog box that follows the file will be the the proper, correct size and you can use it normally.