Filter empty items out of lists

The email template does this already when formatting the body of the
message. But the spreadsheet preview doesn’t, which means you get lists
like:
- thing
- thing
- None

This commit fixes that.

This was a pre-existing bug, but gonna roll it in with this PR.

app/templates/components/table.html

@@ -83,7 +83,9 @@
     {% if text is iterable and text is not string %}
       <ul class="list list-bullet">
         {% for item in text %}
-          <li>{{ item }}</li>
+          {% if item %}
+            <li>{{ item }}</li>
+          {% endif %}
         {% endfor %}
       </ul>
     {% else %}