Add flag to check if toc is empty

[njanakiev]

First of all thanks for the great project! I think an useful feature might be to have a flag to see if the table of contents is empty. Perhaps something in this direction:

{% if not toc.empty %}
<h2>Table of Contents</h2>
{% endif %}

This could be useful for short articles that do not require a table of contents.

[rathboma]

Yes! I definitely need something like this. I have an empty 'on this page' block otherwise.

[markdboyd]

For what it's worth, this seems to work for conditionally rendering the TOC on my pages:

        {% assign toc_content = content | toc_only | strip %}
        {% if toc_content != '' %}
          <h2>Table of Contents</h2>
          <div id="table-of-contents">
            {{ toc_content }}
          </div>
        {% endif %}