Changes

Don't require a validator for loading Vex doctype.
Since there was no validator in Vex, the default doctype (TEI, 
contributed by the TEI plugin) couldn't be loaded. Thus, #22357 occured. 
We don't need a validator in Vex though since validation happens in WST 
anyway.
Fixes #22357.
(Although I wonder why this worked in 3.0)