Invitation Code field cannot be left empty despite default text

Steps to reproduce

1. Manage Users > Invitations
2. Leave field empty and click "Get a new invitation"
3. Error appears saying field must not be empty

What happens?

No invitation code is generated, instead there is an error because the field was left empty.

What is expected?

A random invitation code should be generated as per the default field text "Leave empty for a random code".

Context

Funkwhale version(s) affected: 1.3.0-rc3