patch 8.2.4634: Vim9: cannot initialize a variable to null_list

Problem:    Vim9: cannot initialize a variable to null_list.
Solution:   Give negative count to NEWLIST. (closes #10027)
            Also fix inconsistencies in comparing with null values.
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index 8b9dbd6..24a7f95 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -122,13 +122,13 @@
   END
   v9.CheckDefExecFailure(lines, 'E1131:', 2)
 
-  # Getting variable with NULL blob allocates a new blob at script level
+  # Getting variable with NULL blob fails
   lines =<< trim END
       vim9script
       var b: blob = test_null_blob()
       add(b, 123)
   END
-  v9.CheckScriptSuccess(lines)
+  v9.CheckScriptFailure(lines, 'E1131:', 3)
 enddef
 
 def Test_add_list()