Support or handle runtime deleting of hierarchical white list facet rules properly

A specific edge-case which currently isn't properly handled yet is when at runtime (e.g. in the console) a specific hierarchical white list facet rule is deleted.

The handling in QFacetRuleStateManager.getReferenceUUID(String facetRuleUUID) now returns a null value in two cases:

1. the path for the facetRuleUUID no longer exists (node is deleted, cached uuid entry removed through its processDestroyedId() event handling method), or
2. the node referenced by the facet Rule (jcr:uuid or jcr:path) no longer exists

The current handling in HippoAccessManager.updateReferenceFacetRules() doesn't cater for this distinction and only assumes the 2nd case (logging an error and then continues/ignores the fact).

Naively 'fixing' this by excluding the facet rule (by marking it defunct) for subsequent initializeImplicitReadAccess() however might be dangerous, as potentially this might suddenly 'expose' a far wider (no longer path/hierarchy constrained) read access for current logged in users.

This needs thorough investigation and verification how (or if) we can/should support this.