Description:
------------
There are three methods available to retrieve an attribute - getAttribute, getAttributeNo, getAttributeNs.
It seems to be natural to expect the similar kind of behavior from them in case when no attribute by given parameters is found.
Which is not the case - first one returns NULL, the rest - string(0) "".

Test script:
---------------
<?php
  $reader = new XmlReader();
  $reader->xml('<?xml version="1.0" encoding="UTF-8"?><a y="" z="1"></a>');
  $reader->read();
  var_dump($reader->getAttribute('x'));
  var_dump($reader->getAttributeNs('x', 'http://example.com'));
  var_dump($reader->getAttributeNo(2));
?>

Expected result:
----------------
NULL
NULL
NULL

Actual result:
--------------
NULL
string(0) ""
string(0) ""