StreamWriterContentHandler.getPrefix shouldn't return null if DEFAULT_NS_PREFIX is used

think about the message like
<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope"><soap:Body><add xmlns="http://apache.org/cxf/calculator/types"><arg0>1</arg0><arg1>2</arg1></add></soap:Body></soap:Envelope>
here "add" element has namespace http://apache.org/cxf/calculator/types, but use DEFAULT_NS_PREFIX "", in this case getPrefix should empty string "" , but not null.

Attach a testcase to expose this problem, current code will throw XMLStreamException