WindowServer: Fix switching between shadows and no shadows with themes

We weren't properly handling switching between having a shadow and
not having a shadow when switching themes. This allows an empty string
in the theme configuration for a shadow path, meaning no shadow should
be rendered.

Userland/Services/WindowServer/WindowFrame.h

@@ -90,6 +90,10 @@ public:
     void theme_changed()
     {
         m_dirty = m_shadow_dirty = true;
+        m_top_bottom = nullptr;
+        m_left_right = nullptr;
+        m_bottom_y = m_right_x = 0;
+
         layout_buttons();
         set_button_icons();
     }