Optional Parameters not optional for Soap Calls using WSDL mode

I try to call a webservice provided by SAP to update customer data. I used SoapUI for testing the connection and the actual request that is needed. Everything worked fine, this is how the SoapUI request for changing an email address looks like:

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:urn="urn:sap-com:document:sap:soap:functions:mc-style">
   <soapenv:Header/>
   <soapenv:Body>
      <urn:Ze12RfcMaintainCustomer>     
         <!--Optional:-->
         <PiTEmail>
            <!--Zero or more repetitions:-->
            <item>
               <StdNo>X</StdNo>
               <EMail>me@example.com</EMail>
            </item>
         </PiTEmail>
         <!--Optional:-->
         <PiTEmailX>
            <!--Zero or more repetitions:-->
            <item>
               <StdNo>X</StdNo>
               <EMail>X</EMail>
            </item>
         </PiTEmailX>
         <PieKunnr>4711</PieKunnr>
      </urn:Ze12RfcMaintainCustomer>
   </soapenv:Body>
</soapenv:Envelope>

The webservice accepts a whole lot more parameters, but all of them are optional and not needed for this task.

Now if I try to do the same request in PHP with a SoapClient in WSDL mode, I get an error for every optional parameter that is not included in the request, e.g.:

SOAP-ERROR: Encoding: object hasn't 'EmailSrch' property

This is my code (simplified):

$params = array(
    'PieKunnr' => 4711,
    'PiTEmail' => array(
        'item' => array(
            'StdNo' => 'X',
            'EMail' => 'me@example.com',                    
        ),        
    ),
    'PiTEmailX' => array(
        'item' => array(
            'StdNo' => 'X',
            'EMail' => 'X',                    
        ),        
    ),    
);

$result = $service->Ze12RfcMaintainCustomer($params);

If I put all the optional parameters into the request, it works.

Why can optional parameters not be omitted in SoapClient requests?

It turned out that the WSDL file (which is automatically generated by SAP) did not have the "minOccurs" attribute for alle the elements, although the service endpoint does not require them. I am now using the Soap Client in non-WSDL mode - this seems to be the easiest workaround for my problem.