Allow resource without data (or data that cannot be described with a URI)

[rufuspollock]

As @akariv suggested in https://github.com/frictionlessdata/specs/issues/685#issuecomment-643433683

• Resource path/data - allow a resource without data (or data that cannot be described with a URI)

@akariv note you can already have a "non-uri" e.g. you can have abc. What more would you want to have?

Aside: i don't like the oneof of path and data but i don't know any other approach.