Allow empty edm:Schema elements

The current XML Schema for the EDM namespaces requires the edm:Schema element to contain at least one child element.

This is problematic in at least two use cases:

• OData API Designer tools - when starting with a "blank page"
• Services that "shrink" the schema to match user authorization - when the current user isn't allowed to see anything

Both require workarounds, e.g. having a dummy annotation in the schema telling that "this schema is intentionally empty".