Navigating to /diffusion/identity/ and clicking the Create Identity button will present a form with a single field, "Assigned To". Regardless of leaving this empty or putting in a user account, submitting the form results in an error about the authorPHID column not allowed to be null.
While investigating T15440 it seems that the PhabricatorDiffusionIdentity created by this form is missing a number of fields such that it cannot be persisted to the database. This appears to be an incomplete workflow. The form likely needs to be fleshed out to fill in all the state that makes up an identity. It's possible the current implementation was intended to be a multi-form step, e.g. select the assignee then continue to another form for filling in other details.