Combine({}) returns '' instead of null

[bryantaustin13]

Demonstrated in cql-tests CqlStringOperatorsTest.xml:: CombineEmptyList
The spec at https://cql.hl7.org/09-b-cqlreference.html#combine states "If the source argument is null, or the source list is empty, the result is null."

The return parameter for Combine({}) should be:
"parameter": [
{
"name": "return",
"_valueBoolean": {
"extension": [
{
"url": "http://hl7.org/fhir/StructureDefinition/data-absent-reason",
"valueCode": "unknown"
}
]
}
}
]

instead it is:
"parameter":[
{
"name": "return"
}
]
which is indicating that the engine is creating an empty string.