CachedPrincipalMembershipReader could return empty principal under concurrency

CachedPrincipalMembershipReader can be configured to define a stale time during which an outdated principal list is returned while the cache is being refreshed. 

If a stale time is configured then there will be a period when the cache it's being updated by one request thread (here) and others request could serve an empty list of principals because the property that contains the cached principal list is not written yet.  This can happens depending on the concurrency level and the time it takes to build the principal list from one thread.

Also that could happen only if the property is not present which could only occur the first time the cache is being built as the property is not being deleted afterwards.