Crash when attempting to open menu bar item with action but no menu items assigned

[Velorien]

Describe the bug
When Alt key is pressed and MenuBarItem gets focus, attempting to move focus with left/right arrow keys causes a crash when the target MenuBarItem has an action but Menu property is set to null or an empty array.

This is caused by the framework attempting to open the menu even if it does not exist.

To Reproduce
Steps to reproduce the behavior:

1. Create an application with a MenuBar and at least two MenuBarItems with actions but no MenuItems assigned.
2. Press Alt key or focus the MenuBar by some other means.
3. Press right arrow key
4. Application crashes:

Unhandled exception. System.IndexOutOfRangeException: Index was outside the bounds of the array.
   at Terminal.Gui.MenuBar.OnMenuOpened()
   at Terminal.Gui.MenuBar.set_openCurrentMenu(Menu value)
   at Terminal.Gui.MenuBar.OpenMenu(Int32 index, Int32 sIndex, MenuBarItem subMenu)
   at Terminal.Gui.MenuBar.MoveRight()
   at Terminal.Gui.MenuBar.<.ctor>b__21_1()
   at Terminal.Gui.View.InvokeKeybindings(KeyEvent keyEvent)
   at Terminal.Gui.MenuBar.ProcessKey(KeyEvent kb)
   at Terminal.Gui.View.ProcessHotKey(KeyEvent keyEvent)
   at Terminal.Gui.Application.ProcessKeyEvent(KeyEvent ke)
   at Terminal.Gui.WindowsDriver.ProcessInput(InputRecord inputEvent)
   at Terminal.Gui.WindowsDriver.<PrepareToRun>b__34_0(InputRecord e)
   at Terminal.Gui.WindowsMainLoop.Terminal.Gui.IMainLoopDriver.MainIteration()
   at Terminal.Gui.MainLoop.MainIteration()
   at Terminal.Gui.Application.RunMainLoopIteration(RunState& state, Boolean wait, Boolean& firstIteration)
   at Terminal.Gui.Application.RunLoop(RunState state, Boolean wait)
   at Terminal.Gui.Application.Run(Toplevel view, Func`2 errorHandler)
   at Terminal.Gui.Application.Run(Func`2 errorHandler)
   at Program.<Main>$(String[] args) in REDACTED 105

Minimal repro code

using Terminal.Gui;

Application.Init();

var menu = new MenuBar(
    menus: new[]
    {
        new MenuBarItem { Title = "Test 1", Action = () => { } },
        new MenuBarItem { Title = "Test 2", Action = () => { } },
    });

Application.Top.Add(menu);

Application.Run();
Application.Shutdown();

Expected behavior
Focus is moved without a crash.