Conversion fails on OPF files without a creator

[mwbenowitz]

Currently the webpub conversion service fails if no dc:creator field is present in the metadata block of an ePub's OPF file. This should be allowed according to the ePub 3.2 standard which only requires identifier, title and language.

The comment on the extractMetadataMember states that it will return undefined on failure, but the extractContributors method expects this return to always be an array.

I think this could be handled by simply handling the undefined method but I'm not sure if that's in keeping with the standards for how this was built.