Update SQL query to include bitstreams with NULL descriptions

This change addresses issues https://redmine.dataone.org/issues/4140 for bitstreams not appearing in object list 11/21

Previously, the LIKE operator filter out 'dc_readme' was inadvertently removing records with a NULL description.
PostgreSQL has an IS DISTINCT FROM operator that works as we need for NULLs.
Merge pull request #455 from datadryad/dan-trello-9

Update SQL query to include bitstreams with NULL descriptions