patch 8.2.2781: add() silently skips when adding to null list or blob

Problem:    Add() silently skips when adding to null list or blob.
Solution:   Give an error in Vim9 script.  Allocate blob when it is NULL like
            with list and dict.
diff --git a/src/version.c b/src/version.c
index c3e2ea8..6216faf 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2781,
+/**/
     2780,
 /**/
     2779,