Fix to enable empty table assertion "$db->dontSeeInDatabase('entity');"

I'm doing unit testing and I'm using a combination of [PHPUnit's setUpBeforeClass](http://phpunit.de/manual/3.7/en/fixtures.html#fixtures.sharing-fixture) and [assertPreConditions](http://phpunit.de/manual/3.7/en/fixtures.html#fixtures.examples.TemplateMethodsTest.php). 

So in the ``assertPreConditions`` I would like to assert that a table is empty BEFORE running tests cases. Best way seems to use the Db module ```$this->db->dontSeeInDatabase('entitiesTable');```

So I've done some small modifications to the #select methods so it allows a select without ```$criteria```