PJ Eby added the comment:
This area of the spec is different between 333 and 3333 precisely to clarify this point, without technically changing conformance requirements. PEP 333 was supposed to require the app to supply an argument, while PEP 3333 was changed to encourage the server to allow the app to omit it.
It's an annoying grey area in the spec itself, but not at all a grey area for for the validation tool, which is supposed to validate the strictest interpretation of the spec. A conformant server is allowed to not work with a missing argument, so it's a bad idea for your app to omit it if you want to run on all conformant servers. (And this is entirely independent of whether it's a good idea to read the entire stream in the first place.)