Add test that empty exterior facet set works
Create empty arrays for facet lists rather than None

It is possible that some processes do not own any exterior facets when
running in parallel.  In this case we should still arrange that the type
of the exterior_facet_node_list matches.  To whit, create an empty array
of the appropriate data type, rather than using None.
Merge pull request #373 from firedrakeproject/fix-empty-facet-list

Fix empty facet list