How this happens: a user starts to send a letter job, then in another tab starts a SMS
or email job, the sender_id is set in the session. Then the user goes
back to the letter job tab and creates the job. The sender_id is set in
the metadata of the csv file, and causes an exception when trying to persist
the letter notification.
This PR adds a check to ensure the sender_id is not set for letter jobs.
This will catch a small use case where the user has multiple tabs open
and has started sending an SMS or email job, then tries to send a letter
job.