When generic function is defgeneric'ed, but not defmethod'ed, pcl::generic-function-pretty-arglist retruns nil on it. I doubt that using any other function instead of pcl::generic-function-pretty-arglist will help, because even common-lisp:describe doesn't show any useful information.
Slime with this tiny patch handles this situation better:
Before Slime described such methods as (test I) (where 'I' is (substring "NIL" 1 -1) :)
Now it describes it as (test <no info>).
May be, <no info> is not good string, feel free to replace it with better one :)