How to check if apigee flow variable is empty or null in javascript

[#1]

How to check if apigee flow variable is empty or null in javascript? For example, here I am creating flow variable called screeningReturn from xml payload.

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ExtractVariables async="false" continueOnError="false" enabled="true" name="EVExtractScreeningResults">
    <DisplayName>EV.ExtractScreeningResults</DisplayName>
    <IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>
    <Source clearPayload="false">screeningReturnResponse</Source>
    <XMLPayload stopPayloadProcessing="false">
        <Namespaces/>
        <Variable name="screeningReturn" type="nodeset">
            <XPath>/Results</XPath>
        </Variable>
    </XMLPayload>
</ExtractVariables>

The variable screeningReturn can be null if xml has don't have Results tag. I need to check this in java script. I am using below function to do null or empty check.

 function isEmpty (data) {
	    
	    if(typeof(data) == 'number' || typeof(data) == 'boolean') {
            return false;
        }
        if(typeof(data) == 'undefined' || data === null ) {
            return true;
        }
  
        if(typeof(data.length) != 'undefined') {
            return data.length == 0;
        }
		for (var h in data) {
			if (data.hasOwnProperty(h)) {
				return false;
			}
		}
		return true;
    };

But I am getting following error - TypeError: Cannot find function hasOwnProperty in object com.apigee.flow.Variables@44a8ce54.