Upper('') should return an empty string instead of no value

[bryantaustin13]

Demonstrated in cql-tests CqlStringOperatorsTest.xml:: UpperEmpty

The return parameter for Upper('') should be:
"parameter": [
{
"name": "return",
"valueString" = ""
}
]

instead it is:
"parameter":[
{
"name": "return"
}
]