tile-handler-empty: make it harder to dirty the empty common buffer

When the empty->tile was unreffed because its buffer was,
a tile of an other buffer might become the only 'owner' of
'common_buffer' which was possibly dirtied after unlocking
this tile.

By making every tile_handler_empty->tile sharing its
data with a never ureffed tile it will be harder to legally
dirty the 'common_buffer'