define-method-combination: fix :documentation processing.

Don't store (nil) if there's no :documentation.

src/pcl/defcombin.lisp

@@ -89,13 +89,16 @@
  (documentation (getf (cddr whole) :documentation canary))
  (ioa (getf (cddr whole) :identity-with-one-argument nil))
  (operator
- (getf (cddr whole) :operator type-name)))
- (unless (eql documentation canary)
- (unless (stringp documentation)
-  (%program-error "~@<~S argument to the short form of ~S must be a string.~:>"
-  :documentation 'define-method-combination)))
+  (getf (cddr whole) :operator type-name)))
+ (unless (or (eq documentation canary)
+  (stringp documentation))
+ (%program-error "~@<~S argument to the short form of ~S must be a string.~:>"
+ :documentation 'define-method-combination))
  `(load-short-defcombin
- ',type-name ',operator ',ioa ',documentation (sb-c:source-location))))
+ ',type-name ',operator ',ioa
+ ',(and (neq documentation canary)
+ documentation)
+ (sb-c:source-location))))
 
 (defun load-short-defcombin (type-name operator ioa doc source-location)
  (let ((info (make-method-combination-info