patch 9.0.0837: append() reports failure when not appending anything

Problem:    append() reports failure when not appending anything.
Solution:   Only report failure when appending something. (closes #11498)
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index e9ec439..10ae2ee 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -3721,8 +3721,8 @@
 
       assert_equal(1, setbufline(b, 5, 'x'))
       assert_equal(1, setbufline(b, 5, ['x']))
-      assert_equal(1, setbufline(b, 5, []))
-      assert_equal(1, setbufline(b, 5, test_null_list()))
+      assert_equal(0, setbufline(b, 5, []))
+      assert_equal(0, setbufline(b, 5, test_null_list()))
 
       assert_equal(1, 'x'->setbufline(bufnr('$') + 1, 1))
       assert_equal(1, ['x']->setbufline(bufnr('$') + 1, 1))