Should allow clearing slug (UpdateUser with empty slug)

[MrOrz]

As-is

Currently if the user does not provide slug when editing profile (for example, just changing display names), the operation will be rejected.

To-be

Change UpdateUser

• When slug is exactly an empty string, clear the slug (For edit name dialog)
• When slug argument is not provided at all, leave it as-is. (For avatar dialog)

[Jadepypy]

trying to resolve this issue