Allow NULL "use restriction" fields for located URIs

Allow NULL "use restriction" fields for located URIs

The asset.uri.use_restriction field, which is really a sort of public notes
field for 856 fields, was grabbing the $u subfield (URL) as a sort of last-gasp
effort to give it some data. However, the effect was rather odd and led to
workarounds like Conifer's skin to avoid displaying the use restriction field
if its value was identical to the URL, etc.

Instead, stop grabbing $u and handle the case where use_restriction column is
NULL gracefully, just like the schema intended.

git-svn-id: svn://svn.open-ils.org/ILS/branches/rel_2_0@20125 dcc99617-32d9-48b4-a31d-7c20da2025e4