Description:
------------
I have an xsd that defines an element that can be empty. I can create an object to match OK but if I serialize it it will not de-serialize.

I knwo the example looks a little odd but this is a simplified version of what we do when we construct WSDL for a method that takes no parameters - you get an empty element. 

If you can suggest a better way to code the element definition, one that will not cause this problem to occur, then that would be a satisfactory solution.

Just coding <xs:element name="testTriggerEvent"/> is no better, by the way.

Reproduce code:
---------------
<?php

echo phpversion('sdo') . "\n";
$wsdl = <<<EOF
<?xml version="1.0" encoding="UTF-8"?>
    <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" 
      targetNamespace="http://AnnotationTest">
      <xs:element name="testTriggerEvent">
        <xs:complexType>
          <xs:sequence>
          </xs:sequence>
        </xs:complexType>
      </xs:element>
    </xs:schema>
EOF;

file_put_contents('bug.wsdl',$wsdl);

$xmldas            = SDO_DAS_XML::create('bug.wsdl');
$doc               = $xmldas->createDocument('testTriggerEvent');
$root              = $doc->getRootDataObject();
$serialized_root   = serialize ($root);
$unserialized_root = unserialize($serialized_root);
?>

Expected result:
----------------
I expect the object to de-serialize without any bother and definitely no exception to be thrown.

Actual result:
--------------
1.0.4
<br />
<b>Fatal error</b>:  Uncaught exception 'SDO_Exception' with message 'The referenced object is null ' in C:\Program Files\Apache Group\Apache2\htdocs\MyService\AnnotationTest\bug.php:23
Stack trace:
#0 C:\Program Files\Apache Group\Apache2\htdocs\MyService\AnnotationTest\bug.php(23): unserialize('C:18:&quot;SDO_DataO...')
#1 C:\Program Files\Zend\ZendStudioClient-5.1.0\bin\php5\dummy.php(1): include('C:\Program File...')
#2 {main}
  thrown in <b>C:\Program Files\Apache Group\Apache2\htdocs\MyService\AnnotationTest\bug.php</b> on line <b>23</b><br />