layouts/_default/single.html: Only get imageConfig on local images

Don't try to get image information when the image isn't local. We
generally expect that images are in the local filesystem's static
directory, but if the user has specified a remote image—ie, with
an http:// or https:// prefix—then we shouldn't try imageConfig.

layouts/_default/single.html

@@ -10,11 +10,15 @@
   {{- with .Params.images -}}{{ range first 1 . }}
   "image": {
     "@type": "ImageObject",
-    "url": "{{ . | absURL }}",
-    {{ with (imageConfig (printf "/static/%s" .)) }}
+    "url": "{{ . | absURL }}"
+    {{- /* Don't try to get imageConfig if image param is not local */ -}}
+    {{- if not (or (hasPrefix . "http://") (hasPrefix . "https://")) -}}
+    {{- with (imageConfig (printf "/static/%s" .)) -}}
+    ,
     "height": "{{ .Height }}",
     "width": "{{ .Width }}"
     {{ end }}
+    {{ end }}
   },
   {{- end -}}{{ end }}
   "url": "{{ printf "%s" .Permalink }}",