Is your feature request related to a problem? Please describe.

There is no (public) way to unset a schema after you've set one. Even though the client doesn't distinguish between unset schema and an empty schema, the backend API does.

Describe the solution you'd like

When the set schema is None, remove the whole schema property from the underlying _properties.

Describe alternatives you've considered

We could always send None if a schema is empty, but this wouldn't be right, either.

Additional context

Needed for #9064