Issue in XmlSerializer when serializing the empty statements

[mdanish98]

There is an issue in XmlSerializer, it is not able to set the Empty list of statements inside Operation -> inputVariables -> operationVariable -> value -> Entity.

It still serializes the empty statement. The implementation is already with a comment that specifically mentions statements, maybe it needs to do it in a recursive manner.

The exact location of this implementation is here.

Below is the test to reproduce this issue:

public void serializeOperation() throws SerializationException, SAXException {

        Operation operation = new DefaultOperation.Builder().idShort("dummyOperation").inputVariables(new DefaultOperationVariable.Builder().value(
                new DefaultEntity.Builder().idShort("dummyEntity").build()).build()).build();

        Submodel submodel = new DefaultSubmodel.Builder().id("dummySubmodel").idShort("dummySubmodel")
                .submodelElements(operation).build();

        String xml = new XmlSerializer().write(new DefaultEnvironment.Builder().submodels(submodel).build());
}

Note: We checked just the above scenario, maybe the root cause is different or there could be another scenarios where this issue is reproducible.