This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
From the spec : The combination of authorization constraints that name roles or that imply roles via the name �*� shall yield the union of the role names in the individual constraints as permitted roles. A security constraint that does not contain an authorization constraint shall combine with authorization constraints that name or imply roles to allow unauthenticated access. The special case of an authorization constraint that names no roles shall combine with any other constraints to override their affects and cause access to be precluded.
So an empty constraint will override all others, denying access to the resource.