patch 8.2.0502: Vim9: some code is not tested

Problem:    Vim9: some code is not tested.
Solution:   Add more tests.  Fix uncovered problems.
diff --git a/src/highlight.c b/src/highlight.c
index 5d90e69..9f8bfbc 100644
--- a/src/highlight.c
+++ b/src/highlight.c
@@ -4964,7 +4964,7 @@
 	    semsg(_(e_invarg2), eap->arg);
 	    return;
 	}
-	end = skip_regexp(p + 1, *p, TRUE, NULL);
+	end = skip_regexp(p + 1, *p, TRUE);
 	if (!eap->skip)
 	{
 	    if (*end != NUL && !ends_excmd(*skipwhite(end + 1)))