How to compare variable with NULL value

The problem I’m having

How to compare NULL value passed in macro?

The context of why I’m trying to do this

What I’ve already tried

Check the code below

Some example code or error messages

{% macro m(p1, p2, p3) %}
{% if p3 == '' %}
    v3 = 100
{% else %}
   v3 = p3
{% endif %}
{% endmacro %}