WindowServer: Fix crash when hovering over title buttons

This fixes a crash where if you switched to a theme that has hover
icons for title buttons, then back to a theme that does not. Then
when you next hover over the title buttons the window server would
crash.

This was due to the hover_bitmap multi-scale bitmap pointer being
non-null, but not containing any bitmaps, so hitting an assertion
when painting.

Userland/Services/WindowServer/MultiScaleBitmaps.h

@@ -24,6 +24,7 @@ public:
     Gfx::BitmapFormat format() const { return m_format; }
     bool load(StringView filename, StringView default_filename = {});
     void add_bitmap(int scale_factor, NonnullRefPtr<Gfx::Bitmap>&&);
+    bool is_empty() const { return m_bitmaps.is_empty(); }
 
 private:
     MultiScaleBitmaps() = default;