LP#1927870: MARC Source selector initialization - allow set to null

LP#1927870: MARC Source selector initialization - allow set to null

Allow setting the source selector back to unset on null.

This fixes the issue of paging through results and not having
the source selector get set back to unset when needed.

Testing notes:

You need a results list with a mix of bibs that have the source set
and that have it set to null.

Concerto data by default doesn't have bib source set.

Perform a keword search for 'concertos' and edit every other
bib to set a source of OCLC.
or
update biblio.record_entry set source=1 where id in (22,92,88);

Observe the behavior of the source field before and after the fix.

Before the fix, you will reach a record with the source set, and the source
field will stay set even when paging through records that don't have
it set.

Signed-off-by: Josh Stompro <stomproj@larl.org>
Signed-off-by: Mike Rylander <mrylander@gmail.com>