How to return true value for null list of records

This is the known behaviour of a!isnull() and a!isnullOrEmpty() also, as it probably uses length() function to check if a value is null which would return 1 for a null CDT or record. You can consider it as a best practice to use the below code to check null values for CDTs and Records specially.

{
  a!isNullOrEmpty({}),
  cast(type!Map,{}) = a!map(),
  cast('recordType!{aa4ce38f-1a9d-4827-af51-e2ccb80a2c8b}AS Vehicle',{}) = ri!value,
  cast(type!{urn:com:appian:types:PA}PA_AA_VEHICLE,{}) = ri!cdt
}

The primitive isnull() function is a bit finnicky with respect to what it considers "null" - in Appian, arrays are a totally different data type from single values, and an empty array has never technically counted as null according to isnull().  In older days we were encouraged to set up a separate expression rule like "rule!apn_isBlank()" which would do a more robust test for null / blank / empty, returning a more intiutive return when fed (for example) an empty list of query result, or other sorts of potentially-empty values.  These days however, we have a!isNullOrEmpty() which accomplishes at least (i'll estimate) 80 - 90% of the corner-case handling that we formerly needed to use a specialty expression rule for (there remains a particular corner case, which I don't remember off the top of my head).

I recently wrote a blog post about NULL checks. You find it here: appian.rocks/.../