Fix PmdDescriptor

A PHP warning is displayed when using the PMD format on a project with no violations:
```xml
$ php insight.phar analysis d7d70442-b52c-4072-8e03-45e6a47e1ca2 --format="pmd"
PHP Warning:  Invalid argument supplied for foreach() in phar:///var/lib/jenkins/insight.phar/SensioLabs/Insight/Cli/Descriptor/PmdDescriptor.php on line 31
<?xml version="1.0" encoding="UTF-8"?>
<pmd timestamp="2014-11-16T11:15:54+01:00"/>
```

This PR fix this bug.