How to check for null in Twig?

Depending on what exactly you need:

• is null checks whether the value is null:

   {% if var is null %}
       {# do something #}
   {% endif %}
  

• is defined checks whether the variable is defined:

   {% if var is not defined %}
       {# do something #}
   {% endif %}
  

Additionally the is sameas test, which does a type strict comparison of two values, might be of interest for checking values other than null (like false):

{% if var is sameas(false) %}
    {# do something %}
{% endif %}