patch 8.2.4698: Vim9: script variable has no flag that it was set

Problem:    Vim9: script variable has no flag that it was set.
Solution:   Add a flag that it was set, to avoid giving it a value when used.
            (closes #10088)
diff --git a/src/testdir/test_vim9_builtin.vim b/src/testdir/test_vim9_builtin.vim
index b20ff14..9e60666 100644
--- a/src/testdir/test_vim9_builtin.vim
+++ b/src/testdir/test_vim9_builtin.vim
@@ -153,13 +153,21 @@
   END
   v9.CheckDefExecFailure(lines, 'E1130:', 2)
 
-  # Getting variable with NULL list allocates a new list at script level
+  # Getting an uninitialized variable allocates a new list at script level
+  lines =<< trim END
+      vim9script
+      var l: list<number>
+      add(l, 123)
+  END
+  v9.CheckScriptSuccess(lines)
+
+  # Adding to a variable set to a NULL list fails
   lines =<< trim END
       vim9script
       var l: list<number> = test_null_list()
       add(l, 123)
   END
-  v9.CheckScriptSuccess(lines)
+  v9.CheckScriptFailure(lines, 'E1130:', 3)
 
   lines =<< trim END
       vim9script