CREATE EXTENSION envvar;
SELECT COALESCE(length(get_env('HOME')), 0) >= 0;
 ?column? 
----------
 t
(1 row)

SELECT get_env('no such envvar');
 get_env 
---------
 
(1 row)

SELECT get_env(NULL);
 get_env 
---------
 
(1 row)