Don't add continuation when 2nd defendant is blank

There is case where the blank fields of the defendant model
where being concatenated and added to the PDF form, so that
the continuation sheet was being appended.

This change is a prevention of that. The reason for checking
only of title & full_name if they are blank, is because the
model's validations handle that already. The check is only in
'#as_json'.
Refined the logic for determining if the second defendant is present
Merge pull request #84 from ministryofjustice/67312946/bug-second-defendant-continuation

Don't add continuation when 2nd defendant is blank
Discussed it with Tom, before merging