patch 8.2.0539: comparing two NULL list fails

Problem:    Comparing two NULL list fails.
Solution:   Change the order of comparing two lists.
diff --git a/src/list.c b/src/list.c
index 9fe5384..451e585 100644
--- a/src/list.c
+++ b/src/list.c
@@ -368,12 +368,15 @@
 {
     listitem_T	*item1, *item2;
 
-    if (l1 == NULL || l2 == NULL)
-	return FALSE;
     if (l1 == l2)
 	return TRUE;
     if (list_len(l1) != list_len(l2))
 	return FALSE;
+    if (list_len(l1) == 0)
+	// empty and NULL list are considered equal
+	return TRUE;
+    if (l1 == NULL || l2 == NULL)
+	return FALSE;
 
     range_list_materialize(l1);
     range_list_materialize(l2);