Protect biblio.extract_located_uris() from a NULL uri_use value by using the href...

Protect biblio.extract_located_uris() from a NULL uri_use value by using the href as a fallback

If an 856 field contained a subfield "9" but did not contain a subfield
"z", "2", or "n", then ingest of the MARC record would fail as it attempted
to retrieve an asset.uri row where uri_use = NULL - which was never true.

This change adds the subfield "u" to fall back to for the uri_use variable,
just like we do for the uri_label variable, and prevents ugly ingest errors
from getting thrown back at the client.

git-svn-id: svn://svn.open-ils.org/ILS/trunk@18382 dcc99617-32d9-48b4-a31d-7c20da2025e4