[BUG] Users Can Be Created With a Blank Username

[pladesma]

Describe the bug
You can create users with a blank username via the REST API.

To Reproduce
Steps to reproduce the behavior:

1. Open the client API (i.e. https://localhost:8443/api) and log in
2. Navigate to Users > POST /users
3. Click "Try it out"
4. In the Request body, make the username blank
5. Execute the request

Expected behavior
The API returns an error and no new user is created

Actual behavior
The API returns a success response and a new user is created with a blank username