Bug 29687: Uninitialized warning C4/XSLT line 286

Bug 29687: Uninitialized warning C4/XSLT line 286

From plack-opac-error.log:
[WARN] Use of uninitialized value $value in concatenation (.) or string at /usr/share/koha/C4/XSLT.pm line 286.

Test plan:
An opac search triggered the warning. So repeat it without warns.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit 74382c321c16348919f83f63e505dea93239c2ca)

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>