Use seo_description instead of page.description

seo_description is used in the meta tags but not in the ld+json
If page.description is not set, the the description at ld+json will be null.

lib/template.html

@@ -155,8 +155,8 @@
     "datePublished": {{ page.date | date_to_xmlschema | jsonify }},
 {% endif %}
 
-{% if page.description %}
-    "description": {{ page.description | jsonify }},
+{% if seo_description %}
+    "description": {{ seo_description | jsonify }},
 {% endif %}
 
 {% if site.logo %}