send stream_empty when tts streaming has nothing to play

lib/session/call-session.js

@@ -3095,6 +3095,9 @@ Duration=${duration} `
   }
 
   _onTtsStreamingEmpty() {
+    this.requestor?.request('tts:streaming-event', '/streaming-event', {event_type: 'stream_empty'})
+      .catch((err) => this.logger.info({err}, 'CallSession:_onTtsStreamingEmpty - Error sending'));
+
     const task = this.currentTask;
     if (task && TaskName.Say === task.name) {
       task.notifyTtsStreamIsEmpty();