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>

C4/XSLT.pm

@@ -286,6 +286,7 @@ sub XSLTParse4Display {
 
     my $varxml = "<variables>\n";
     while (my ($key, $value) = each %$variables) {
+        $value //= q{};
         $varxml .= "<variable name=\"$key\">$value</variable>\n";
     }
     $varxml .= "</variables>\n";