xmlreader: getAttributeNo()/getAttributeNs() return null, not an empty string

[lacatoire]

Both descriptions said they return an empty string when the attribute is missing or the reader is not positioned on an element node. They actually return null.

Verified in php-src: the stubs declare ?string, and in ext/xmlreader/php_xmlreader.c retchar starts at NULL and return_value is only written via RETVAL_STRING when the attribute exists, so it stays null otherwise.