Fix Lost Wake Bug in ROSOutAppender (#2033)

This fixes a bug in ROSOutAppender that consists of waiting on the condition_variable `queue_condition_` without checking if the `log_queue_` is empty.

In this situation if the `log_queue_` had some messages that were inserted while `ROSOutAppender::logThread` was publishing other messages, the new messages in the queue won't be published, until another message eventually is added.

Note that the `notify_all` sent in the destructor would not cause the unpublished messages to get published, as when the `queue_condition_`  is awaken by this notification, `shutting_down_` would be true and would cause `ROSOutAppender::logThread`  to return immediately.

Co-authored-by: Adel Fakih <adel.fakih@avidbots.com>

clients/roscpp/src/libros/rosout_appender.cpp

@@ -137,7 +137,10 @@ void ROSOutAppender::logThread()
         return;
       }
 
-      queue_condition_.wait(lock);
+      if (log_queue_.empty())
+      {
+        queue_condition_.wait(lock);
+      }
 
       if (shutting_down_)
       {