Description:
------------
A call like addAttribute("attrname", "") results in
"PHP Warning:  SimpleXMLElement::addAttribute(): Attribute name and value are required". In addition to the warning, the attribute is discarded.


Reproduce code:
---------------
<?php
$xml = new SimpleXmlElement("<img></img>");
$xml->addAttribute("src", "foo");
$xml->addAttribute("alt", "");
echo $xml->asXML()."\n";
?>


Expected result:
----------------
<?xml version="1.0"?>
<img src="foo" alt=""/>


Actual result:
--------------
PHP Warning:  SimpleXMLElement::addAttribute(): Attribute name and value are required in [...]/test.php on line 4
<?xml version="1.0"?>
<img src="foo"/>