transformer.setMuleContext(muleContext);
transformer.setReturnSourceIfNull(true);
ExpressionConfig config = new ExpressionConfig("null", "groovy", null);
ExpressionArgument argument = new ExpressionArgument("test", config, false);
argument.setMuleContext(muleContext);
transformer.addArgument(argument);
MuleMessage message = new DefaultMuleMessage("Test", muleContext);
Object result = transformer.transformMessage(message, null);
assertTrue(result instanceof MuleMessage);
MuleMessage transformedMessage = (MuleMessage) result;