MINOR: event_hdl: add event_hdl_async_equeue_isempty() function

Add event_hdl_async_equeue_isempty() to check is the event queue is
empty from an advanced async task handler.

If 68e692da0 ("MINOR: event_hdl: add event handler base api")
is being backported, then this commit should be backported with it.
diff --git a/include/haproxy/event_hdl.h b/include/haproxy/event_hdl.h
index 2770d1d..6b396ad 100644
--- a/include/haproxy/event_hdl.h
+++ b/include/haproxy/event_hdl.h
@@ -425,6 +425,12 @@
 	return MT_LIST_POP(queue, struct event_hdl_async_event *, mt_list);
 }
 
+/* use this for advanced async mode to check if the event queue is empty */
+static inline int event_hdl_async_equeue_isempty(event_hdl_async_equeue *queue)
+{
+	return MT_LIST_ISEMPTY(queue);
+}
+
 /* use this to initialize <sub_list> event subscription list */
 void event_hdl_sub_list_init(event_hdl_sub_list *sub_list);