lambda list checking optimizes away dispatch macros which return nil

[Mark Evenson]

After [r12693][1], dispatch macros which return nil are stripped from processing in lambda lists.

For example

CL-USER> (defun foo (&key (this :that))
           this)

CL-USER> (foo :this (#+:nil :other))
Odd number of keyword arguments.
   [Condition of type PROGRAM-ERROR]

Among other things, this causes [usocket][2] to fail to load, erroring with the slightly mysterious "Unrecognized keyword argument :NAME" as reported by Kevin Raison.

[1]: ​http://trac.common-lisp.net/armedbear/changeset/12693

[2]: ​http://common-lisp.net/project/usocket/

[ehuelsmann]

(In [12766]) Fix #101: Regression in 0.20 where dispatch macros return NIL.