Prevent {NULL} values from blocking bib saving

Prevent {NULL} values from blocking bib saving

With both the old ARRAY_ACCUM and ARRAY_AGG, and with array_nulls on,
PostgreSQL 9.0 on Debian Squeeze was generating values of '{NULL}'
rather than actual nulls. This would in turn prevent bibs without ISBNs
or ISSNs from being saved successfully. A CASE statement protects
against that problem and returns a clean NULL.

Signed-off-by: Dan Scott <dscott@laurentian.ca>
Signed-off-by: Ben Shum <bshum@biblio.org>