Bug 26669: Avoid a warn in update_sql

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>