It is sometimes necessary to differentiate between a variable that has "empty contents" and one which is actually "non-existent". This filter is here to help you. It is applied to the #ENV{thing} tag and makes it possible to test to see if the "thing" variable has been declared for the current template environment.
[(#ENV{thing}|is_null)]
This code returns "1" of the variable "thing" is null (i.e. non-existent) or nothing if it is not null (i.e. existent) [1].
This filter can not be applied to variables declared with #SET where the value is not present in the environment but only in the template.