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 %}

Also if your variable is an ARRAY, there are few options too:

{% if arrayVariable[0] is defined %} 
    #if variable is not null#
{% endif %}

OR

{% if arrayVariable|length > 0 %} 
    #if variable is not null# 
{% endif %}

This will only works if your array is defined AND is NULL