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/vim9execute.c b/src/vim9execute.c
index 4e1af4e..60b58f9 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -1020,6 +1020,10 @@
 	    if (tv->vval.v_dict == NULL)
 		(void)rettv_dict_alloc(tv);
 	    break;
+	case VAR_BLOB:
+	    if (tv->vval.v_blob == NULL)
+		(void)rettv_blob_alloc(tv);
+	    break;
 	default:
 	    break;
     }