Allow null for username field in request payloads

I'd like to be able to send a payload that consists of a user JSON that looks like:

{
  "active": false,
  "expirationDate": "2020-12-07T05:00:00.000Z",
  "externalSystemId": "0732272"
}

Currently, this fails validation because username must not be null.

In that example, I'm trying to deactivate the user because they're no longer at our institution. The feed where this data is coming from doesn't have the information that allows me to construct a username, it literally only has the data for an externalSystemId.

(In an ideal world, I'd just use the deactivateMissingUsers flag, but our feeds consist of ~70k users, so I have to import users in batches which means that all users not in the last batch would be deactivated.)