Fixes symfony 2.6+ debug toolbar from throwing exception

This adds a check in DataCollector/GuzzleDataCollector::getRequests() method to check that the 'requests' offset exists in the data array before returning it. It also adds a test to cover it. 

In symfony 2.6+ when no requests are made, this throws an illegal offset exception. 

Reported in #66 .
It seems that the `requests` key exists, but contains a `null` value (see the failing tests). Using `isset()` instead should fix that. However, it might be good to also test if the data really is an array when it is not `null`.