If you want to check whether or not documents (attachments) have been added to a database variable, you can by using the document method:
{% if custom.attachements.things.document == blank %}
{% ic %}
{::infotext as="hover"}
{% t "Please add the missing invoices as PDF" %}
{:/infotext}
{% endic %}
{% endif %}
{% input custom.attachements.things as:file_collection %}
Output:
