Description:
------------
Current asXML() behavior of the SimpleXMLElement object is not really neat: this method may accept "filename" as a first argument for data storing on a HDD. 

However, if a "filename" is empty (let's say "null" or empty string), this method will throw a warning about it.

This logic asks from the programmer create very strange structures to avoid such warning messages: "if ($filename) { return $xml->asXML($filename); } else { return $xml->asXML(); }"

Test script:
---------------
<?php

function example($file_path = null)
{
	$xml = new \SimpleXMLElement('<data></data>');

	return $xml->asXML($file_path);

}

example(); # will throw "PHP Warning:  SimpleXMLElement::asXML(): Filename cannot be empty in ..."

Expected result:
----------------
I suggest to make a "filename" as an optional argument for this method.

Actual result:
--------------
PHP Warning: SimpleXMLElement::asXML(): Filename cannot be empty in ...