patch 8.2.3459: Vim9: need more tests for empty string arguments

Problem:    Vim9: need more tests for empty string arguments.
Solution:   Add more tests.  Also use empty argument with menu_info() to get
            the top-level menu names. (Yegappan Lakshmanan, closes #8925)
diff --git a/src/testdir/test_menu.vim b/src/testdir/test_menu.vim
index 3df1560..c867162 100644
--- a/src/testdir/test_menu.vim
+++ b/src/testdir/test_menu.vim
@@ -407,6 +407,9 @@
         \ shortcut: '', modes: ' ', submenus: ['menu']},
         \ menu_info(']Test'))
   unmenu ]Test
+
+  " Test for getting all the top-level menu names
+  call assert_notequal(menu_info('').submenus, [])
 endfunc
 
 " Test for <special> keyword in a menu with 'cpo' containing '<'