There are examples in DBLP where the corresponding field (ee) contains different urls, e.g. http://dblp.uni-trier.de/rec/bibtex/ms/Hoffmann2008 points to his thesis and have no doi.

Therefore, one would have to test whether the ee field contains a doi or some other url. For a doi we can (I guess) replace it with doi. This should exist in some BibTeX and hopefelly land in the doi field of zotero. For another url we should replace it with url.