patch 8.2.4526: Vim9: cannot set variables to a null value

Problem:    Vim9: cannot set variables to a null value.
Solution:   Add null_list, null_job, etc.
diff --git a/src/evalvars.c b/src/evalvars.c
index 44882ca..4f7252c 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -999,6 +999,11 @@
     listitem_T	*item;
     typval_T	ltv;
 
+    if (tv->v_type == VAR_VOID)
+    {
+	emsg(_(e_cannot_use_void_value));
+	return FAIL;
+    }
     if (*arg != '[')
     {
 	// ":let var = expr" or ":for var in list"