[PATCH] tpl/embedded: Make Open Graph's series optional

cherry-picked from gohugoio/hugo@b82b547

Author: razonyang

layouts/partials/templates/opengraph.html

@@ -41,12 +41,14 @@
 {{- /* If it is part of a series, link to related articles */}}
 {{- $permalink := .Permalink }}
 {{- $siteSeries := site.Taxonomies.series }}
+{{- if $siteSeries }}
 {{ with .Params.series }}{{- range $name := . }}
   {{- $series := index $siteSeries ($name | urlize) }}
   {{- range $page := first 6 $series.Pages }}
     {{- if ne $page.Permalink $permalink }}<meta property="og:see_also" content="{{ $page.Permalink }}" />{{ end }}
   {{- end }}
 {{ end }}{{ end }}
+{{- end }}
 
 {{- /* Facebook Page Admin ID for Domain Insights */}}
 {{- with site.Social.facebook_admin }}<meta property="fb:admins" content="{{ . }}" />{{ end }}