I had this problem as well and couldn't find a clear answer online. However, I do have a a ringtone that is 100% silent that I use on my phone for unwanted callers. A search for Richard K Miller and Silence should help.
It took some digging but I found the location of the Mac's ringtones (System ▸ Library ▸ PrivateFrameworks ▸ ToneLibrary.framework ▸ Versions ▸ A ▸ Resources ▸ Ringtones).
I renamed an unused ringtone to filename -backup.m4u, copied my silence.m4u to the folder and then renamed that to the name of the ringtone I just made a back up of.
Example:
renname constellation.m4u to constellation -backup.m4u
copy silence.m4u to folder
rename silence.m4u to constellation.m4u
set that has as the ringtone in the facetime settings
This is a kludge of a fix that shouldn't exist. A simple checkbox in the FT UI settings would have prevented such customization.