Bug fix to OPAC shelf browsing query statement

Bug fix to OPAC shelf browsing query statement

If the items.location field was NULL, then the current
SQL query would produce no results.  I have turned this
into a conditional block that removes the location condition
in the query if the location is not specified.

In addition, there was a small change to opac-detail.tmpl
that changed Library to Shelves when the shelf browser was
open.  This removes a potential redundant Library Library
display if Library is contained in the starting_homebranch.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>
Signed-off-by: Henri-Damien LAURENT <henridamien.laurent@biblibre.com>