Distinct on a list of all nulls does not return a list but a single null

[bryantaustin13]

To dupe:
Run this test:

<?xml version="1.0" encoding="utf-8"?>
<tests xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://hl7.org/fhirpath/tests" xsi:schemaLocation="http://hl7.org/fhirpath/tests ../../testSchema/testSchema.xsd"
	   name="CqlListOperatorsTest">
	<capability code="interval-operators"/>
	<group name="SingleTest" version="1.0">
		<test name="DistinctNullNullNull" version="1.0">
			<expression>distinct { null, null, null}</expression>
			<output>{ null }</output>
		</test>
	</group>
</tests>

Actual return: null

I am wondering if the serialization is causing the problem.
Instead of a list of null like a single null is returned. According to https://cql.hl7.org/2017Jul/09-b-cqlreference.html?#distinct
"The distinct operator returns the given list with duplicates eliminated."
The description is a little ambiguous, because it also states "If the argument is null, the result is null."