patch 9.0.0457: substitute prompt does not highlight an empty match

Problem:    Substitute prompt does not highlight an empty match.
Solution:   Highlight at least one character.
diff --git a/src/version.c b/src/version.c
index 85e5a9d..019abf3 100644
--- a/src/version.c
+++ b/src/version.c
@@ -704,6 +704,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    457,
+/**/
     456,
 /**/
     455,