adb generated code doesn't handle empty elements

The deserialize code generated by wsdl2c doesn't correctly handle elements that have no content and treats them as if they weren't present at all. The attached patch fixes this. The patch also includes a number of changes to handle const strings more correctly which were needed to deal with the use of the zero length string constant. Also, it fixes the prototypes of the <foo>_is_particle functions so the code can compile with warnings turned on.