pkp/pkp-lib#9295 DOI issue less case on export

[touhidurabir]

for pkp/pkp-lib#9295 .

[asmecher]

@ewhanson, tagging you for a quick look at this. It's in support of continuous (issueless) publication; essentially when a published article doesn't have an issue associated with it, we omit the issue element in the CrossRef XML. Does that match CrossRef's expectations for this use case?

[ewhanson]

@asmecher, looks good to me. Did a check and and journal_issue is an optional element and the metadata passes Crossref's validation check without that element, so this should be good to go from that perspective.