arvo: allow the empty desk (%$) in scries

+slaw fails to parse empty strings as %tas, so we special-case it here.

pkg/arvo/sys/arvo.hoon

@@ -191,7 +191,7 @@
   ?.  ?=({@ @ @ @ *} u.pux)  ~
   =+  :*  hyr=(slaw %tas i.u.pux)
           fal=(slaw %p i.t.u.pux)
-          dyc=(slaw %tas i.t.t.u.pux)
+          dyc=?~(i.t.t.u.pux (some %$) (slaw %tas i.t.t.u.pux))
           ved=(slay i.t.t.t.u.pux)
           tyl=t.t.t.t.u.pux
       ==