Bug 8772 - Uninitialized variable triggers error log entry in smart_rules.pl

Bug 8772 - Uninitialized variable triggers error log entry in smart_rules.pl

Changed declartion from:
  my $op = $input->param('op');
to
  my $op = $input->param('op') || q{};
in order to give an empty string default preventing error
logs from filling needlessly.

Signed-off-by: Marc Veron <veron@veron.ch>
The patch removes warnings like:
Use of uninitialized value $op in string eq at...

However there is one more warning, but it is not related to $op (addressed by this patch):
smart-rules.pl: Use of uninitialized value in string ne at .../admin/smart-rules.pl line 388.

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>