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/ex_eval.c b/src/ex_eval.c
index 70b52a3..72da2f2 100644
--- a/src/ex_eval.c
+++ b/src/ex_eval.c
@@ -1527,7 +1527,7 @@
     else
     {
 	pat = eap->arg + 1;
-	end = skip_regexp(pat, *eap->arg, TRUE, NULL);
+	end = skip_regexp(pat, *eap->arg, TRUE);
     }
 
     if (!give_up)