Avoid emitting links to non-existent plugin tag pages.

templates/doc-page.html

@@ -53,11 +53,14 @@
 
       {{ this.body }}
       {% if this.parent._slug == 'events' %}
+        {% if this._slug in site.query('/plugins').distinct('tags') %}
           <br>
           <p>
-          <a href="{{ ('/plugins/tag/' ~ this._slug ~ '/')|url }}" class="ref">Plugins That Use This Event</a>
+            A list of some plugins that make use of this event is available
+            <a href="{{ ('/plugins/tag/' ~ this._slug ~ '/')|url }}" class="ref">here</a>.
           </p>
         {% endif %}
+      {% endif %}
 
       {% if this.version_history %}
       <div class="version-info version-info-collapsed">