Relation fallback if no category has been selected
I had to use a bunch of conditionals to accomplish this - it's not ideal but here's what I came up with

{% set category = craft.app.request.getParam("category") %}
{% set function = craft.app.request.getParam("function") %}
{% set industry = craft.app.request.getParam("industries") %}

{% if not category | length %}
        {% set category = "blog,caseStudies,videos,webinar,whitePapers" %}
{% endif %}

{% set category = category | split(',') %}


{% set posts = craft.entries({
    section: category,
    orderBy: "postDate desc"
}) %}

  {% if function | length %}
    {% set functionSection = craft.categories.group('businessFunction').slug( function|split(',') ) %}
    {% set posts = posts.relatedTo(functionSection) %}
  {% endif %}

  {% if industry | length %}
    {% set industrySection = craft.categories.group('industry').slug( industry|split(',') ) %}
    {% set posts = posts.relatedTo(industrySection) %}
  {% endif %}