Prevent attempted DOI registration with no <creator> element

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.
Merge pull request #488 from datadryad/fix_ezid_missing_creator

Prevent attempted DOI registration with no <creator> element