wayland/surface: Only call destroy func when available

When calling gdk_wayland_surface_export_handle(), if we pass
some 'user_data' but no 'destroy_func', GTK4 crashes. That's
because in xdg_exported_handle() we are unconditionally calling
destroy_func -- even when it's NULL.

Fix that by checking if there's a destroy function before calling

Fixes #2179
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