Added a isset() to avoid notices on certain PHP configurations.

Using the Zendesk API client with Laravel causes an E_NOTICE when using the Tickets::findAll methods due to a direct access to the ```$params['ccd']``` variable.

Wrapping the test in ```isset``` fixed the problem.

I could not test this change using the Zendesk PHPUnit test suite because all tests fail or are skipped due to an authentication failure.