Provide alternate background if no image

Ref #33

web/themes/custom/geofresco/src/templates/layout/ds-1col.html.twig

@@ -13,6 +13,12 @@
 
   {{ title_suffix.contextual_links }}
 
-  {{ ds_content }}
+  {% set image = ds_content.field_image|render %}
+  {% if image %}
+    {{ image }}
+  {% else %}
+    <figure class="background-replacement-for-no-image image"></figure>
+  {% endif %}
+  {{ ds_content|without('field_image') }}
 
 </{{ ds_content_wrapper }}>