Only create json-file if email was succesfully sent

[vincentjanv]

Right now, it will create an empty file before sending a mail (if it does not already exist for an invoice). But if sending fails, for whatever reason, nothing will be written to the JSON file. After retrying, it will try to parse the (empty) JSON-file but this will fail, resulting in the email never being sent.

I will create a PR to fix this.