Use empty tag for img thumbs

apps/course/templates/course/_items.html

@@ -8,13 +8,11 @@
     {% if course.is_deferred_start %}data-future-course data-future-course-time={{ course.deferred_start_at.timestamp }}{% endif %}
 >
 <a class="courses__preview" href="{% if course.status == 0 %}{% url 'course_edit' course.id %}{% else %}{% url 'course' course.id %}?next={{ request.get_full_path }}{% endif %}">
-  {% if course.cover %}
   {% thumbnail course.cover.image "300x170" crop="center" as im %}
   <img class="courses__pic" src="{{ im.url }}" width="{{ im.width }}" height="{{ im.height }}"/>
-  {% endthumbnail %}
-  {% else %}
+  {% empty %}
   <img class="courses__pic" src="{% static 'img/no_cover.png' %}" width="300px" height="170px"/>
-  {% endif %}
+  {% endthumbnail %}
   <div class="courses__view">Подробнее</div>
   {% if course.is_featured %}
   <div class="courses__label courses__label_fav"></div>