Bug 10656: (follow-up) handle OPAC sorting of authvals where lib_opac is NULL

Bug 10656: (follow-up) handle OPAC sorting of authvals where lib_opac is NULL

The OPAC description for an authorized value is not required to be
populated.  In particular, if it is NULL, the staff description is
displayed instead.

This patch makes sure that the sort order (in OPAC mode) uses either
the staff description or the OPAC description as needed for each
value.

To test:

[1] Make sure that AdvancedSearchTypes includes "ccode"
[1] Arrange your CCODE values so the sort order for staff labels
    is different from the sort order for OPAC descriptions.  Also,
    ensure that one of the OPAC descriptions is NULL.  For example,

    authorised_value | lib     | lib_opac
    --------------------------------------
    ZZZ              | A_STAFF | Z_PUBLIC
    DDD              | D_STAFF | NULL
    AAA              | Z_STAFF | A_PUBLIC

[2] Prior to the patch, any CCODE values where the OPAC description
    is NULL will sort first in the OPAC advanced search page, even
    if the displayed label shouldn't come first.
[3] Apply the patch.
[4] Verify that the collection list on the OPAC advanced search page
    is now correct.
[5] Verify that the sort order on the staff advanced search page
    has not changed.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Works nicely, tested in staff and OPAC.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 0c67e94c8b8dbe1912fd5ba4b503cf53789b75a5)
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>