FEEL considers null part of the value type domain. Also, most aggregate functions like all(), any(), sum(), etc will return null if any of the elements in the input are null.
This ticket requests the implementation of alternate aggregate functions that ignore null values in the input. The goal is to support CQL (1) semantics as well as make it easier for users that deal with sparse inputs.
(1) CQL reference: https://cql.hl7.org/09-b-cqlreference.html#aggregate-functions