cell: handle NULL cso vertex_elements state object on bind

cell: handle NULL cso vertex_elements state object on bind

drivers need to handle NULL cso vertex elements (and others) objects.
It is possible the cso code saves/restores NULL objects (if no normal
cso object was bound before this was invoked).
This led to segfaults (for example demos/cubemap) for apps which were using
things like creating mipmaps before drawing anything.