patch 8.1.0455: checking for empty quickfix stack is not consistent

Problem:    Checking for empty quickfix stack is not consistent.
Solution:   Use qf_stack_empty(). (Yegappan Lakshmanan)
diff --git a/src/version.c b/src/version.c
index caf7168..d4c872a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -793,6 +793,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    455,
+/**/
     454,
 /**/
     453,