How to omit undefined vars in filters?

Hi!
I have a syntax question.
When putting variable in module’s parameter, you can omit it if it is undefined, like so:

`
parameter: {{ variable | default(omit) }}

`

But how to do the same thing with list filters where either var can be undefined?

`
parameter: {{ list1 | difference(list2) }}

`

How to make module work regardless of whether list1 or list2 defined or not?