fix: url-less authors should not have empty links (#1410)

The url field from authors.yml is optional, so no link should be
build when the url is empty or not defined.

Fix #1403

_layouts/post.html

@@ -61,7 +61,11 @@ tail_includes:
           <em>
             {% if authors %}
               {% for author in authors %}
+                {% if site.data.authors[author].url -%}
                   <a href="{{ site.data.authors[author].url }}">{{ site.data.authors[author].name }}</a>
+                {%- else -%}
+                  {{ site.data.authors[author].name }}
+                {%- endif %}
                 {% unless forloop.last %}{{ '</em>, <em>' }}{% endunless %}
               {% endfor %}
             {% else %}