Inform when a property to retain is empty or not existing

As a developer or an administrator, I am warned when I am producing partially empty records

AC

• When a configured property is not part of the schemas held by a given document about to become a record, it is ignored and a mention is added to the logs as INFO "Property schema:property is not available in document [docid]. It cannot be put under retention."
• When a configured property is empty in a document about to become a record, it is ignored and a mention is added to the logs as WARN "Property schema:property is empty in document [docid]. No file to put under retention."
• When an entry of a multivalued configured property is empty in a document about to become a record, it is ignored and a mention is added to the logs as WARN "Retention: entry [number] in schema:property is empty in document [docid]. No file to put under retention."