Bug 26669: Avoid a warn in update_sql

In C4/Reports/Guided.pm update_sql() called by test suite return warn :
Use of uninitialized value $sql in substitution (s///)

Test plan :
Run prove t/db_dependent/Reports/Guided.t and see warning disapearing
(whouchhhh)

Signed-off-by: Séverine Queune <severine.queune@bulac.fr>

Signed-off-by: Séverine Queune <severine.queune@bulac.fr>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>

C4/Reports/Guided.pm

@@ -656,7 +656,7 @@ sub update_sql {
     my $cache_expiry = $fields->{cache_expiry};
     my $public = $fields->{public};
 
-    $sql =~ s/(\s*\;\s*)$//;    # removes trailing whitespace and /;/
+    $sql =~ s/(\s*\;\s*)$// if defined $sql;    # removes trailing whitespace and /;/
     my $report = Koha::Reports->find($id);
     $report->last_modified(dt_from_string);
     $report->savedsql($sql);