knowledge-post.hbs: check for date.

There's one case where date is set to `null`.

Author: XhmikosR

layouts/knowledge-post.hbs

@@ -11,7 +11,9 @@
       <article>
         <div class="blogpost-header">
           <h1>{{title}}</h1>
-          <span class="blogpost-meta">{{#if author}}{{site.by}} {{author}}, {{/if}}<time datetime="{{ date }}">{{ strftime date }}</time></span>
+          <span class="blogpost-meta">{{#if author}}{{site.by}} {{author}}, {{/if}}
+          {{#if date}}<time datetime="{{ date }}">{{ strftime date }}</time>{{/if}}
+          </span>
         </div>
 
         {{{ contents }}}