TWIG: is defined and is not null

You need to declare the var in each check so {% if var is defined and var is not null %}.

Wrong

{% if var is not null and var is defined %}

This does not work because a variable which is null in twig is defnied, but if you check for null first and it is not defined it will throw an error.

Correct

{% if var is defined and var is not null %}

This will work because we check if it is defined first and will abord when not. Only if the variable is defined we check if it is null.

I have always used the ?? operator to provide a 'default' value when I know a variable might not be defined. So {% if (var is defined and var is not null) %} is equivalent to:

{% if (var ?? null) is not null %}

If you just want to check whether a value that might not be defined is truthy you can do this:

{% if (var ?? null) %}