[%pS migration] Use %pS in ext/xmlreader - #23174

Usefulness analysis:

• in xmlreader_write_property() the class name can contain null bytes (https://3v4l.org/EiAoh#v) so %pS is needed; property name cannot contain null bytes because the only values are those present in xmlreader_prop_handlers and none of those have null bytes, but is updated for consistency
• in XMLReader::expand() the class name can contain null bytes (https://3v4l.org/G91cW#v) so %pS is needed