Do not show tags and category in side bar if metadata field is empty

It is possible if user do not set any value in it. Ideally he should
remove the metadata field but he may miss to do it.

templates/article.html

@@ -84,11 +84,11 @@
             <time pubdate="pubdate" datetime="{{ article.date.isoformat() }}">{{ article.date.strftime('%b') }} {{ day }} {{- article.date.strftime(', %Y') }}</time>
             {% endif %}
             {% include 'last_updated.html' %}
-            {% if article.category %}
+            {% if article.category|trim|count > 0 %}
             <h4>Category</h4>
             <a class="category-link" href="{{ SITEURL }}/categories.html#{{ category.slug }}-ref">{{ article.category }}</a>
             {% endif %}
-            {% if article.tags %}
+            {% if article.tags and article.tags[0]|trim|count > 0 %}
             <h4>Tags</h4>
             <ul class="list-of-tags tags-in-article">
                 {% for tag in article.tags|sort %}