Add missing parameter

I think the code was broken before and guess this is the correct fix
What is this fixing? [MongoCollection::remove()](http://php.net/manual/en/mongocollection.remove.php)'s first parameter defaults to an empty array.

Cross-reference to the (actual implementation](https://github.com/mongodb/mongo-php-driver/blob/9edce80dd25db5323b90cc8ee16a5758c0122a00/collection.c#L1620), where the first argument is parsed as optional (if it is provided, we validate that it's an array or object). In turn, this function invokes [php_mongocollection_remove()](https://github.com/mongodb/mongo-php-driver/blob/9edce80dd25db5323b90cc8ee16a5758c0122a00/collection.c#L1529), which defaults it to an empty array if it's null.