MDEV-21343 Threadpool/Unix- wait_begin() function does not wake/create threads, when it should

Fixed the condition for waking up/creating another thread.

If there is some work to do (if the request queue is not empty),
a thread should be woken or created.

The condition was incorrect since 18c9b345

sql/threadpool_generic.cc

@@ -1339,7 +1339,7 @@ void wait_begin(thread_group_t *thread_group)
   DBUG_ASSERT(thread_group->connection_count > 0);
 
   if ((thread_group->active_thread_count == 0) && 
-     (is_queue_empty(thread_group) || !thread_group->listener))
+     (!is_queue_empty(thread_group) || !thread_group->listener))
   {
     /* 
       Group might stall while this thread waits, thus wake