There will likely already be a silent audio file in your skin folder that you can make a copy of, which will be 0 seconds and less than 100bytes. Some common uses of this so you can find it is in "count" and "nightcore" files. If you dont have one at all somehow or its more convenient, try the one listed at the bottom of: wiki/en/Guides/Custom_Hitsound_Library#silence
Once you've got one of those, simply rename it to "combobreak", go back to osu! and press ctrl+shift+alt+s to reload your skin, and you're good to go
That being said, don't try to use any 0-byte file. It doesn't even have a header, causing the file to not read properly. At least there's a 44-byte file you can see in the link they posted.