Description:
------------
When using a Web Service and passing it a PHP object that 
contains NULL fields, these fields end up as empty tags in the 
message

Reproduce code:
---------------
no easy code, but when using a web service that expects a document style message, e.g.
<foo>
  <bar>...</bar>
</foo>

And passing an object
class Foo {
  public $bar = 'hello';
  public $baz;
}

The XML created is something like:
...
<SOAP-ENV:Body>
  <message-name>
    <foo>
       <bar>Hello</bar>
       <baz/>
    </foo>
  </message-name>
</SOAP-ENV:Body>


Expected result:
----------------
The message should be:

<SOAP-ENV:Body>
  <message-name>
    <foo>
       <bar>Hello</bar>
    </foo>
  </message-name>
</SOAP-ENV:Body>

Actual result:
--------------
The message is:

<SOAP-ENV:Body>
  <message-name>
    <foo>
       <bar>Hello</bar>
       <baz/>
    </foo>
  </message-name>
</SOAP-ENV:Body>