publication: Make abstract optional (#382)

layouts/publication/single.html

@@ -22,8 +22,10 @@
     <img src="{{ "/img/" | relURL }}{{ .Params.image }}" class="pub-banner" itemprop="image">
     {{end}}
 
+    {{ if .Params.abstract }}
     <h3>{{ i18n "abstract" }}</h3>
     <p class="pub-abstract" itemprop="text">{{ .Params.abstract | markdownify }}</p>
+    {{ end }}
 
     {{ if (.Params.publication_types) and (ne (index .Params.publication_types 0) "0") }}
     <div class="row">