Fix Workflow Index Rebuild ACL Handling

The workflow index rebuild process tried to load and set the ACLs set
during the workflow execution by loading those as files from the
working file directory. This will nearly always fail for terminated
workflows since the ACLs do not exist any longer.

While this has no real consequence since the ACLs will be overwritten
later in the re-indexing process by ACLs from the asset manager anyway,
it still produces a huge amount of NullPointerException stack-traces
(one for each workflow ever run by the system), making it impossible
to spot and identify real errors in the process.