Store AIP: handle AIPs without pointer files
Store AIP: handle AIPs without pointer files

This is needed to support uncompressed AIPs, which do not have
pointer files.

refs #6589

artefactual/archivematica-storage-service,artefactual/archivematica-storage-service,artefactual/archivematica-storage-service,artefactual/archivematica-storage-service