layer shell: attaching null buffer does not behave as expected

[MaxVerevkin]

Expected behavior

Attaching a null buffer to a layer surface unmaps it.

Unmapping a layer_surface means that the surface cannot be shown by the compositor until it is explicitly mapped again. The layer_surface returns to the state it had right after layer_shell.get_layer_surface. The client can re-map the surface by performing a commit without any buffer attached, waiting for a configure event and handling it as usual.

Actual behavior

After attaching null buffer the surface disappears, but the "exclusive zone" is not shifted. Reattaching a non-null buffer does not trigger a configure event.