Fix error for null theme.open_graph.options (theme-next#470)

Author: ljcbaby

_config.yml

@@ -77,7 +77,6 @@ creative_commons:
 open_graph:
   enable: true
   options:
-    type: website
     #twitter_card: <twitter:card>
     #twitter_id: <twitter:creator>
     #twitter_site: <twitter:site>

layout/_partials/head/head-unique.njk

@@ -1,5 +1,9 @@
 {%- if theme.open_graph.enable %}
-  {{ open_graph(theme.open_graph.options) }}
+  {%- if theme.open_graph.options %}
+    {{ open_graph(theme.open_graph.options) }}
+  {%- else %}
+    {{ open_graph() }}
+  {%- endif %}
 {%- endif %}
 
 {# https://github.com/theme-next/hexo-theme-next/issues/866 #}