fix(publication): DOI link not shown if it's the only link

Fix #1340

layouts/partials/page_links_div.html

@@ -15,7 +15,7 @@
   {{ $cite_link = true }}
 {{ end }}
 
-{{ if $cite_link | or $pdf_link | or .Params.external_link | or .Params.url_preprint | or .Params.url_pdf | or .Params.url_slides | or .Params.url_video | or .Params.url_source | or .Params.url_code | or .Params.url_dataset | or .Params.url_poster | or .Params.url_project | or .Params.links | or .Params.projects | or .Params.slides }}
+{{ if $cite_link | or $pdf_link | or .Params.external_link | or .Params.url_preprint | or .Params.url_pdf | or .Params.url_slides | or .Params.url_video | or .Params.url_source | or .Params.url_code | or .Params.url_dataset | or .Params.url_poster | or .Params.url_project | or .Params.doi | or .Params.links | or .Params.projects | or .Params.slides }}
 <div class="btn-links mb-3">
   {{ with .Params.external_link }}
   <a class="btn btn-outline-primary my-1" href="{{ . }}" target="_blank" rel="noopener">{{ i18n "open_project_site" }}</a>