WSO2 EI: empty fields ("") are added as null in the payload transformation - wso2

Question

I'm retrieving some data's from DSS by using select query and then some transformation by payload factory after that I'm passing it to an endpoint, but when I do that the empty fields are being passed as null and not as empty strings ""
"department":"{department":null}",
"Selling_dealer__c":"{Selling_dealer__c":null}"
I wish it really showed like this:
"department":"",
"Selling_dealer__c":""
Note: Also tried the synapse properties file method but not working as excepted refer link 1 refer link 2
<sequence name="LaravelConnectorSequence" xmlns="http://ws.apache.org/ns/synapse">
<payloadFactory media-type="json">
<format>
{
"screen_type": "opportunity",
"vehicle_type__c": "",
"description": "",
"audio_file_name__c": "$1",
"lead_type__c": "Phone",
"request_type__c": "",
"mobile__c": "$2",
"leadsource": "Novum",
"company__c": "N/A",
"location__c": "$3",
"department__c": "$4",
"selling_dealer__c":"$5"
}
</format>
<args>
<arg evaluator="xml"
expression="get-property('audioFileName')"
literal="false" xmlns:ns="http://org.apache.synapse/xsd"/>
<arg evaluator="xml" expression="get-property('mobile')"
literal="false" xmlns:ns="http://org.apache.synapse/xsd"/>
<arg evaluator="xml" expression="get-property('location')"
literal="false" xmlns:ns="http://org.apache.synapse/xsd"/>
<arg evaluator="xml" expression="get-property('department')"
literal="false" xmlns:ns="http://org.apache.synapse/xsd"/>
<arg evaluator="xml"
expression="get-property('selling_dealer__c')"
literal="false" xmlns:ns="http://org.apache.synapse/xsd"/>
</args>
</payloadFactory>
<log level="custom">
<property expression="json-eval($)"
name="=======Laravel Request====" xmlns:ns="http://org.apache.synapse/xsd"/>
</log>
<property expression="get-property('Novum-configs')" name="config"
scope="axis2" type="OM" xmlns:ns="http://org.apache.synapse/xsd"/>
<property expression="$axis2:config//*[local-name()='laravelToken']"
name="Authorization" scope="transport" type="STRING" xmlns:ns="http://org.apache.synapse/xsd"/>
<property expression="$axis2:config//*[local-name()='laravelURL']"
name="uri.var.laravelapi" scope="default" type="STRING" xmlns:ns="http://org.apache.synapse/xsd"/>
<call>
<endpoint>
<http method="POST" uri-template="{uri.var.laravelapi}"/>
</endpoint>
</call>
<log level="custom">
<property expression="json-eval($)"
name="=======Laravel Response====" xmlns:ns="http://org.apache.synapse/xsd"/>
</log>

Answer 1

Since you want to convert your null values into "" strings, one workaround would be to convert the JSON to XML and then extract the elements from that and add them to your final JSON payload. In WSO2 there are different options you can set when converting JSON payloads. You can read about different options and how to work with JSON messages from here.
Since you want null to be "". You need to set the following property. If you are on MI this needs to go into deployment.toml, if on EI this goes into synapse.properties
'synapse.commons.enableXmlNullForEmptyElement'=false
Note: In MI (Surround the property name with single quotes)
When you are extracting data from the DSS response make sure to use Xpath expressions instead of JSON path expressions. Following is a simplified version of your configurations. Note I have hardcoded the DSS response.
<?xml version="1.0" encoding="UTF-8"?>
<api context="/api" name="RESTApi" xmlns="http://ws.apache.org/ns/synapse">
<resource methods="GET" url-mapping="/sample">
<inSequence>
<payloadFactory media-type="json">
<format>{"Selling_dealer__c": null}</format>
<args/>
</payloadFactory>
<property expression="//Selling_dealer__c" name="logInJson" scope="default" type="STRING"/>
<payloadFactory media-type="json">
<format>
{
"screen_type": "opportunity",
"selling_dealer__c": "$1"
}
</format>
<args>
<arg evaluator="xml" expression="$ctx:logInJson"/>
</args>
</payloadFactory>
<property name="messageType" scope="axis2" type="STRING" value="application/json"/>
<respond/>
</inSequence>
<outSequence/>
<faultSequence/>
</resource>
</api>
Above will produce the following response.
{
"screen_type": "opportunity",
"selling_dealer__c": "{"Selling_dealer__c":""}"
}

Answer 2

I used the script mediator to overcome this issue.
<script language="js"><![CDATA[var log=mc.getServiceLog();
var dep = mc.getProperty('department').toString();
var sell = mc.getProperty('selling_dealer__c').toString();
//log.info("===DepartmentValue===: "+dep);
var deps = mc.getProperty('department').startsWith("<Department");
//log.info("++++++++++++++++++++TRUE VALUE++++++++++++++++++"+deps);
if(dep.startsWith("<Department"))
{
dep ="";
mc.setProperty('department',dep);
}
//log.info("===DepartmentValueAfter===: "+dep)
//log.info("===SellingDealerValue===: "+sell);
var sells = mc.getProperty('selling_dealer__c').startsWith("<selling_dealer__c");
//log.info("++++++++++++++++++++TRUE VALUE++++++++++++++++++"+sells);
if(sell.startsWith("<selling_dealer__c"))
{
sell ="";
mc.setProperty('selling_dealer__c',sell);
}
//log.info("===selling_dealer__cValueAfter===: "+sell)
]]></script>