Avoid a crash on doom doctor caused by lexical-let

`lexical-let` is loaded with cl which is not loaded by doom doctor.
So the mapping is scoped with a condition to avoid a void-variable error.

config.org

@@ -445,9 +445,9 @@ after giving an argument."""
   (lexical-let ((inner-id id))
     #'(lambda () (interactive) (chicken/insert-user-character inner-id))))
 
-(map!  ; Add which-key description
-      :leader                           ; Use leader key from now on
-      :desc "Misc character insert" "iM" (chicken/create-user-character-inserter 'misc))
+(map! (:when (boundp 'lexical-let)
+        :leader ; Use leader key from now on
+        :desc "Misc character insert" "iM" (chicken/create-user-character-inserter 'misc)))
 #+END_SRC
 ** TODO Pragmata's own characters
 * Unsorted Functions