[BUG] There is group_name more dash in the generated id

[savoygu]

reproduction link: develop branch https://github.com/savoygu/sass-bem-next/tree/develop
reproduction steps:

• npm install
• gulp sassdoc

Search for undefined-- in the index.html of the generated gh-pages directory. Normally it should be undefined-.
This is the code that produces this result:

{% for item in items %}
   {% include "../includes/partials/item.html.njk" %}
{% endfor %}

If you replace {% include "../includes/partials/item.html.njk" %} with item.html.njk, the result is normal.