Prevent attempted DOI registration with no <creator> element

Should fix the following error

> A Datacite EZID registration was not successfully completed, due to the following reason:
> 
> ERROR: BAD REQUEST - ELEMENT 'DATACITE': [XML] XML ERROR: CVC-COMPLEX-TYPE.2.4.A: INVALID CONTENT WAS FOUND STARTING WITH ELEMENT 'TITLES'. ONE OF '{"HTTP://DATACITE.ORG/SCHEMA/KERNEL-2.2":CREATORS}' IS EXPECTED.

as well as subsequent

> error: bad request - no such identifier

errors for the same item

DIM2DATACITE.xsl would only generate a <creator> block if the item's metadata has a dc.contributor.author.
The datacite schema http://schema.datacite.org/meta/kernel-2.2/metadata.xsd rejects this, so DOIs were not registered for files and subsequent updates failed.

For files with no dc.contributor.author field, (:unav) is provided, meaning value unavailable, possibly unknown.