Fix DEFSTRUCT with (:conc-name nil).
Fix DEFSTRUCT with (:conc-name nil).

It was using "NIL" as the conc-name before.

AugustusHuang/Mezzano,AugustusHuang/Mezzano,froggey/Mezzano,rvedam/Mezzano,varjagg/Mezzano,froggey/Mezzano,rvedam/Mezzano,varjagg/Mezzano