fix: allow PLUGINS to be None

change code to not assume PLUGINS is an iterable, and allow
for its default value of None

Author: osvenskan

templates/base.html

@@ -5,7 +5,7 @@
         <meta http-equiv="X-UA-Compatible" content="IE=edge">
         <meta name="viewport" content="width=device-width, initial-scale=1.0">
         {% block head_links %}
-        {% if 'assets' in PLUGINS %}
+        {% if PLUGINS and 'assets' in PLUGINS %}
         {% include '_includes/minify_css.html' with context %}
         {% else %}
         <link rel="stylesheet" type="text/css" href="{{ SITEURL }}/theme/css/elegant.prod.9e9d5ce754.css" media="screen">

templates/search.html

@@ -39,7 +39,7 @@
 {% endblock content %}
 
 {% block script %}
-    {% if 'tipue_search' in PLUGINS %}
+    {% if PLUGINS and 'tipue_search' in PLUGINS %}
         <script src="{{ SITEURL }}/tipuesearch_content.js"></script>
     {% endif %}
 {{ super() }}