dbug: avoid printing nothing at all

Previously, if filtering subscriptions turned up no results, nothing was
printed. With this change, we explicitly print "no matching subscriptions"
instead.

pkg/arvo/lib/dbug.hoon

@@ -47,6 +47,9 @@
       (ream grab.dbug)
     ::
         %incoming
+      =;  =tang
+        ?^  tang  tang
+        [%leaf "no matching subscriptions"]~
       %+  murn
         %+  sort  ~(tap by sup.bowl)
         |=  [[* a=[=ship =path]] [* b=[=ship =path]]]
@@ -66,6 +69,9 @@
       ==
     ::
         %outgoing
+      =;  =tang
+        ?^  tang  tang
+        [%leaf "no matching subscriptions"]~
       %+  murn
         %+  sort  ~(tap by wex.bowl)
         |=  [[[a=wire *] *] [[b=wire *] *]]