patch 8.1.0651: :args \"foo works like :args without argument

Problem:    :args \"foo works like :args without argument.
Solution:   Fix check for empty argument. (closes #3728)

src/version.c

@@ -799,6 +799,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    651,
 /**/
     650,
 /**/