xdg-surface: an unmap commit is incorrectly treated as initial

1. A client performs a NULL buffer commit for a mapped surface
2. wlr_surface.events.unmap is emitted
3. reset_xdg_surface() is called
4. sent_initial_configure is set to false
5. Role commit handler is called
6. sent_initial_configure is false so an initial configure is sent

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IMO wlroots should avoid sending configures on its own.