bug 7198: show patron name in overdue list even if name column is null

bug 7198: show patron name in overdue list even if name column is null

If the surname or firstname of a patron is NULL, the MySQL concat()
function (properly) returns NULL, leading to the patron name not
displaying in the overdue report.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
(cherry picked from commit 18e06ecf0bfeeefd08575d0512891bea52bc1bde)

Signed-off-by: Chris Nighswonger <chris.nighswonger@gmail.com>