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/userfunc.c b/src/userfunc.c
index 62003a8..12d582a 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -2330,7 +2330,7 @@
      */
     if (*eap->arg == '/')
     {
-	p = skip_regexp(eap->arg + 1, '/', TRUE, NULL);
+	p = skip_regexp(eap->arg + 1, '/', TRUE);
 	if (!eap->skip)
 	{
 	    regmatch_T	regmatch;