[scene] Fix segfault in KWin::WindowPixmap::shape

From scene.h:-

    /**
     * @returns the surface this WindowPixmap references, might be @c null.
     */
    KWaylandServer::SurfaceInterface *surface() const;

There is a situation where subSurface() can return a non-null value, but surface() returns null. This MR just checks that surface() is defined before using it avoiding a segfault.

Reproducer:

• Open Konsole (probably works with any window)
• Right click the decoration and select "Configure Special Window Settings..."
• Close the window specific settings dialog
• kwin_wayland crashes.

BUG: 426567