oslc:describes on ResourceShape should be optional in ReSpec

The ResourceShapes specifications indicates that oslc:describes is an optional property of ResourceShape (Zero-or-many) and that if it is unspecified, then the shape can apply to any resource.

This is what we should use to capture the OSLC common properties - a oslc:CommonProperties ResouceShape that doesn't represent anything, therefore not implying there is some root OSLC resource type.

The Shape Checker only creates a warning on this, and that's probably the right thing to do.

However, ReSpec cannot process a ResourceShape if oslc:describes is not specified or refers to an RDF class that is not defined in any vocabulary (e.g.,foaf:Person).