add a default value for sendmail_path

lib/IdPAccountManager/WebRequest.pm

@@ -358,7 +358,8 @@ sub req_generate_token {
 
     my $sender    = $self->{configuration}->{_}->{notice_from};
     my $recipient = $self->{in}->{email_address};
-    my $sendmail  = $self->{configuration}->{_}->{sendmail_path};
+    my $sendmail  = $self->{configuration}->{_}->{sendmail_path} ||
+                    '/usr/sbin/sendmail';
 
     open(my $handle, '|-', "$sendmail -f $sender $recipient") or do {
         push @{ $self->{out}->{errors} }, "mail_notification_error";