Fix blank site.teaser bug

- Use `assign` instead of capture and then check if variable is `nil` or not.
- Close #412

_includes/archive-single.html

@@ -3,7 +3,7 @@
 {% if post.header.teaser %}
   {% capture teaser %}{{ post.header.teaser }}{% endcapture %}
 {% else %}
-  {% capture teaser %}{{ site.teaser }}{% endcapture %}
+  {% assign teaser = site.teaser %}
 {% endif %}
 
 {% if post.id %}
@@ -14,7 +14,7 @@
 
 <div class="{{ include.type | default: "list" }}__item">
   <article class="archive__item" itemscope itemtype="http://schema.org/CreativeWork">
-    {% if include.type == "grid" and teaser != blank %}
+    {% if include.type == "grid" and teaser %}
       <div class="archive__item-teaser">
         <img src=
           {% if teaser contains "://" %}