IssueXML.php: skip DOCTYPE (fixes cobertura)

[croensch]

After adding Cobertura XML report to PHPUnit my phpcb crashed with:

PHP Fatal error: Uncaught TypeError: DOMNode::appendChild(): Argument #1 ($node) must be of type DOMNode, bool given in /var/lib/jenkins/.composer/vendor/mayflower/php-codebrowser/src/PHPCodeBrowser/IssueXML.php:158

This patch skips '<!DOCTYPE coverage SYSTEM ...' (cobertura), which is a DOMNode that can't be imported, and uses the following element instead.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE coverage SYSTEM "http://cobertura.sourceforge.net/xml/coverage-04.dtd">
<coverage line-rate="0.71621621621622" branch-rate="0" lines-covered="371" lines-valid="518" branches-covered="0" branches-valid="0" complexity="264" version="0.4" timestamp="1709828073">

Might want to also check for false but i think this patch is already deterministic and nice.

[mmoll]

@croensch Thanks, do you need a 4.0.1 release with this included?

[croensch]

@mmoll would be nice but no rush as i only use it optionally in my build