Mailserver schema requires attachment if attachments is present, but SimpleJavaMail always generates empty collection "attachments"

[klauswr]

Citrus Version
3.4.0

Expected behavior
Accept empty collection as "no attachment"

Actual behavior
Error: Failed to marshal object - the content of element attachment is not complete

Caused by: com.consol.citrus.exceptions.CitrusRuntimeException: Failed to marshal object graph
	at com.consol.citrus.mail.model.MailMarshaller.marshal(MailMarshaller.java:120)
	at com.consol.citrus.mail.message.MailMessage.getPayload(MailMessage.java:290)
	at com.consol.citrus.util.MessageUtils.hasXmlPayload(MessageUtils.java:44)
	at com.consol.citrus.validation.xml.DomXmlMessageValidator.supportsMessageType(DomXmlMessageValidator.java:596)
	at com.consol.citrus.validation.xml.XpathMessageValidator.supportsMessageType(XpathMessageValidator.java:161)
	at com.consol.citrus.validation.MessageValidatorRegistry.findMessageValidators(MessageValidatorRegistry.java:72)
	at com.consol.citrus.actions.ReceiveMessageAction.validateMessage(ReceiveMessageAction.java:263)
	at com.consol.citrus.actions.ReceiveMessageAction.doExecute(ReceiveMessageAction.java:168)
	at com.consol.citrus.actions.AbstractTestAction.execute(AbstractTestAction.java:59)
	at com.consol.citrus.DefaultTestCase.executeAction(DefaultTestCase.java:136)
	... 35 more
Caused by: javax.xml.bind.MarshalException
 - with linked exception:
[org.xml.sax.SAXParseException; lineNumber: 0; columnNumber: 0; cvc-complex-type.2.4.b: The content of element 'attachment' is not complete. One of '{"http://www.citrusframework.org/schema/mail/message":attachments, "http://www.citrusframework.org/schema/mail/message":fileName}' is expected.]
	at com.sun.xml.bind.v2.runtime.MarshallerImpl.write(MarshallerImpl.java:301)
	at com.sun.xml.bind.v2.runtime.MarshallerImpl.marshal(MarshallerImpl.java:226)
	at com.consol.citrus.xml.Jaxb2Marshaller.marshal(Jaxb2Marshaller.java:105)
	at com.consol.citrus.mail.model.MailMarshaller.marshal(MailMarshaller.java:118)
	... 44 more

Test case sample
Java: (SimpleJavaMail)
...

        Email email = EmailBuilder.startingBlank()
                .to(toColl)
                .withSubject("subject")
                .withPlainText("text")
                .withHTMLText("html")
                .buildEmail();

...
Citrus:

    @Test
    @CitrusTest
    public void test() {
...
        receive(receiveMessageBuilder -> receiveMessageBuilder
                .endpoint(mailServer)
                .message(MailMessage.request()
                    .from("todo-report@example.org")
                    .to("users@example.org")
                    .cc("")
                    .bcc("")
                    .subject("ToDo report")
                    .body("Hello, world!", "text/plain; charset=us-ascii"))
                .header(CitrusMailMessageHeaders.MAIL_SUBJECT, "ToDo report"));