DolphinQt2: Don't show invalid games in game list

Regression from 1f1dae3.

This problem doesn't happen in DolphinWX as far as I know, but if
you've ran into the problem in DolphinQt2, it will carry over to
DolphinWX because of the shared game list cache.

Source/Core/UICommon/GameFileCache.h

@@ -35,6 +35,7 @@ public:
 
   void Clear();
 
+  // Returns nullptr if the file is invalid.
   std::shared_ptr<const GameFile> AddOrGet(const std::string& path, bool* cache_changed,
                                            const Core::TitleDatabase& title_database);