Fix null reference in recommendations

[HebaruSan]

Problem

When loading the recommendations screen when it would contain multiple options:

System.NullReferenceException: Object reference not set to an instance of an object.
   at CKAN.ChooseRecommendedMods.RecommendedModsListView_ItemChecked(Object sender, EventArgs e)
   at System.Windows.Forms.ListView.OnItemChecked(ItemCheckedEventArgs e)
   at System.Windows.Forms.ListView.WmReflectNotify(Message& m)
   at System.Windows.Forms.ListView.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

I have no idea how I didn't encounter this during development of #2981, since that inherently required loading this screen with multiple options to test. Maybe there's a race condition?

Cause

Apparently ListView.Items can contain null, specifically in the ItemChecked event while processing an AddRange call. Upon reflection this is less surprising than at first blush, since on Windows these are wrappers around C++ components, which probably:

1. Expand the Items list with nulls, since it may be a std::vector that has to copy its contents every time it is expanded
2. Assign them one by one, triggering ItemChecked for each in turn

(Whereas I would expect a C# component to approach this as foreach (...) { Items.Add(...); }, which would never insert a null value.)

The code from #2981 assumed ListView.Items would not contain nulls.

Changes

Now RecommendedModsListView_ItemChecked skips nulls.