Fix JSON serialization issue with double-commas

… I forgot to add an output to YASON:ENCODE!

src/activity-vocabulary.lisp

@@ -280,7 +280,8 @@ into one. Otherwise, parse it into an associative list."
 ;;; ————————————————————————————————————————
 ;; Note-worthy: See the above-defined DEFINE-YASON-ENCODE-SLOTS.
 (defmethod yason:encode ((obj object) &optional (stream *standard-output*))
-  (yason:encode-object obj))
+  (yason:with-output (stream)
+    (yason:encode-object obj)))
 
 (defmethod yason:encode-object ((obj object))
   (typecase *@context*