patch 8.2.1821: Vim9: concatenating to a NULL list doesn't work

Problem:    Vim9: concatenating to a NULL list doesn't work.
Solution:   Handle a NULL list like an empty list. (closes #7064)
diff --git a/src/version.c b/src/version.c
index 8e2115e..8dabd60 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1821,
+/**/
     1820,
 /**/
     1819,