patch 8.2.1731: Vim9: cannot use += to append to empty NULL list

Problem:    Vim9: cannot use += to append to empty NULL list.
Solution:   Copy the list instead of extending it. (closes #6998)
diff --git a/src/version.c b/src/version.c
index 655e349..9e7825a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1731,
+/**/
     1730,
 /**/
     1729,