Make the transfer status fields nullable

[bjester]

Summary

• Prevents Django setting "" default for new transfer status related fields

TODO

• Have tests been updated for the new code?
• Has documentation been written/updated?
• New dependencies (if any) added to requirements file

Reviewer guidance

Tested both migration pathways, with previous migration already applied and without any migrations

Issues addressed

List the issues solved or partly solved by the PR

Documentation

If the PR has documentation, link the file here (either .rst in your repo or if built on Read The Docs)

[jamalex]

Testing both upgrades pathways, which you did, was my main question. Besides that, I couldn't see anything from a skim of the rest of the code the seemed to rely on these fields being str instances (mostly truthiness or equivalence), and the tests all pass, so looks good to me! Thanks!