Fix source payload not preserving in call mediator

When there isn't a defined source when using source and target with call
mediator the source supposed to be the body and after mediation the
original payload in this case source should be preserved but in this
case there isn't a define source it was not happening and I have fixed
it by adding a dummy source.

Fixes: wso2/product-micro-integrator#3086

modules/core/src/main/java/org/apache/synapse/config/xml/CallMediatorFactory.java

@@ -109,6 +109,11 @@ public Mediator createSpecificMediator(OMElement elem, Properties properties) {
 
         OMElement targetEle = elem.getFirstChildWithName(TARGET_Q);
         if (targetEle != null) {
+            if (sourceEle == null) {
+                Source source = CallMediatorEnrichUtil.createSourceWithBody();
+                callMediator.setSourceAvailable(true);
+                callMediator.setSourceForOutboundPayload(source);
+            }
             Target target = new Target();
             populateTarget(callMediator, target, targetEle);
             callMediator.setTargetForInboundPayload(target);