Fix compatibility issue with php 7.2

[ddeath]

Hi I found that this package is not working on php 7.2 because it will throw an error:

local.ERROR: ErrorException: count(): Parameter must be an array or an object that implements Countable in .../vendor/rebing/graphql-laravel/src/Rebing/GraphQL/GraphQLController.php:21

The reason why this happen is that on php 7.1 will count behave different:

count(NULL); // 0

but on php 7.2:

count(NULL); // PHP Warning:  count(): Parameter must be an array or an object that implements Countable