handle missing biblio elements better
handle missing biblio elements better

fixes getting doi from crossref when start with pmid and doi isn't listed in pubmed (e.g. 11457506)

Impactstory/total-impact-core,total-impact/total-impact-core,Impactstory/total-impact-core,total-impact/total-impact-core,total-impact/total-impact-core,total-impact/total-impact-core,Impactstory/total-impact-core,Impactstory/total-impact-core