patch 8.2.3747: cannot remove highlight from an existing sign

Problem:    Cannot remove highlight from an existing sign. (James McCoy)
Solution:   Only reject empty argument for a new sign.
diff --git a/src/version.c b/src/version.c
index 0d00f20..e7b0b2e 100644
--- a/src/version.c
+++ b/src/version.c
@@ -754,6 +754,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3747,
+/**/
     3746,
 /**/
     3745,