bug 10728: fix additional log noise generated by subscription-renew.pl

To test:

[1] Use the renew link to bring up the subscription renewal form.
[2] Verify that a warning message containing the text
    'Problem = a value of 1 has been passed to param without key'
    was not added to the Apache error log.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>

serials/subscription-renew.pl

@@ -60,7 +60,7 @@ my $query = new CGI;
 my $dbh   = C4::Context->dbh;
 
 my $mode           = $query->param('mode') || q{};
-my $op             = $query->param('op') || q{};
+my $op             = $query->param('op') || 'display';
 my $subscriptionid = $query->param('subscriptionid');
 my $done = 0;    # for after form has been submitted
 my ( $template, $loggedinuser, $cookie ) = get_template_and_user(