dbug: avoid printing nothing at all
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.
urbit/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,jfranklin9000/urbit,ngzax/urbit,ngzax/urbit,jfranklin9000/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,ngzax/urbit,jfranklin9000/urbit,jfranklin9000/urbit,urbit/urbit,jfranklin9000/urbit,urbit/urbit,jfranklin9000/urbit,jfranklin9000/urbit