When playing DOOM games, STF is automatic and already built into the game - if there isn't an SBARINFO loaded that says "use something other than STF", the game will use STF by default. For games other than DOOM, there is no built-in faces so any SBARINFO WAD must include it's own face graphics.
This is how it was
before I got involved so I didn't need to change anything on that score, which is why there is nothing about it in the diff
Since there is no "" option for DrawMugShot, what I did do was change the code for all texture drawing things (not only faces) so that it does not crash if the named graphics don't exist, but just does not draw anything where the face would be. This way, if somebody wants to make an SBARINFO that can work with all games -- on condition that somewhere in the list of WADs that are loaded at the time there is player class faces (or old-skool SKIN faces) for the player classes that are in use -- they can just specify "STF" anyway; in this case "STF" is just being used as a placeholder for "whatever face applies here".
For this reason, it might be useful to allow an empty <default> parameter (that is, "") to DrawMugShot since it is now a meaningful thing to do.