Make event field nullable

[gregnewman]

What's this PR do?

Makes event field nullable for Transfer model. actions.transfers.sync_transfer allows the event argument to be None which is necessary in order to save a Transfer that was manually created and not received from a webhook.

Where should the reviewer start?

How should this be manually tested?

Any background context you want to provide?

What ticket or issue # does this fix?

Issues: #315

Definition of Done (check if considered and/or addressed):

• [] Does this contain breaking changes? No
• [] Does this add new dependencies? No
• [] Will this feature require a new piece of infrastructure be implemented? No
• [] Did you update internal documentation appropriately? Yes