According to the ws - security policy specification 1.1 , 5.1.1 Signed Parts Assertion
This assertion specifies the parts of the message that need integrity protection. If no child elements are specified, all message headers targeted at the UltimateReceiver role [SOAP12] or actor [SOAP11] and the body of the message MUST be integrity protected.
So for an empty signed parts element, we have to sign all the message headers. At current we don't sign any header if signed parts element is empty.