Doen't contain:
{% assign input = "A,B,C" | split:"," %}
This prints nothing:
{% unless input contains 'A' %}No A{% endunless %}
This prints "No Z":
{% unless input contains 'Z' %}No Z{% endunless %}
Both states:
{% if collection.tags contains 'tag' %}
loop over tags
{% else %}
don't loop over stuff
{% endif %}