Fix mmistakes#3668 breaking "disable per-page when globally enabled" (mmistakes#3669)

* Fix mmistakes#3668 breaking "disable per-page when globally enabled"

* `default:` filter doesn't fit here

https://shopify.github.io/liquid/filters/default/

Author: iBug

_layouts/single.html

@@ -8,7 +8,11 @@
   {% include page__hero_video.html %}
 {% endif %}
 
-{% if page.url != "/" and site.breadcrumbs or page.breadcrumbs %}
+{% assign breadcrumbs_enabled = site.breadcrumbs %}
+{% if page.breadcrumbs != null %}
+  {% assign breadcrumbs_enabled = page.breadcrumbs %}
+{% endif %}
+{% if page.url != "/" and breadcrumbs_enabled %}
   {% unless paginator %}
     {% include breadcrumbs.html %}
   {% endunless %}