send at least one empty discovery message if have previously announced universes

protocol/sacn/source.cpp

@@ -35,6 +35,7 @@ Source::Source(UUID::uuid cid)
     : Component(cid)
     , discovery_future_(discovery_exitSignal_.get_future())
     , discovery_worker_(&Source::discovery_loop_, this)
+    , sendNullDiscovery(false)
 {
   fctn_ = "OpenLCP sACN Source";
 }
@@ -105,7 +106,14 @@ void Source::discoveryAnnounce()
   /// > sending E1.31 Universe Discovery Packets until such time that it
   /// > resumes transmission of E1.31 Data and/or Synchronization information.
   if (universes_.empty())
-    return;
+    {
+      if (sendNullDiscovery)
+        sendNullDiscovery = false;
+      else
+        return;
+    }
+  else
+    sendNullDiscovery = true;
 
   // framing layer header
   auto frmheader = std::make_shared<EXTENDED::discovery_header>();

protocol/sacn/source.h

@@ -80,6 +80,10 @@ private:
     std::future<void> discovery_future_;
     std::thread discovery_worker_;
     void discovery_loop_();
+
+    /// After terminating all universes, at least one empty universe
+    /// discovery message should be sent.
+    bool sendNullDiscovery;
 };