It should indeed break. If I try this, SOAP UI receives an incomplete response:
<?xml version="1.0" ?><soap:Envelope xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:tns="http://www.example.com/"><soap:Body><tns:OperationResponse><EntityA><Attribute>2</Attribute><EntityB_EntityA><EntityB
and the console of the modeler reads:
com.mendix.integration.WebserviceException: Attribute 'Attribute1' is neither optional nor nillable but mapped value 'MyFirstModule.EntityB.MyFirstModule.EntityB.Attribute1' is null for operation 'Operation'