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)

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    455,
 /**/
     454,
 /**/