Fix for uncaught RuntimeError when retrieving some PMID entries without PMCID#38

caufieldjh merged 7 commits intomainlinkml/linkml-reference-validator:mainfrom

37-runtimeerror-couldnt-resolve-exlinksrv2-the-address-table-is-emptylinkml/linkml-reference-validator:37-runtimeerror-couldnt-resolve-exlinksrv2-the-address-table-is-emptyCopy head branch name to clipboard