Fixing PHPStan issue

Author: moufmouf

src/ConstraintViolationException.php

@@ -15,7 +15,7 @@ class ConstraintViolationException extends Exception implements GraphQLException
 
     public function __construct(ConstraintViolationInterface $violation)
     {
-        parent::__construct($violation->getMessage(), 400);
+        parent::__construct((string) $violation->getMessage(), 400);
         $this->violation = $violation;
     }
 

src/Mappers/Parameters/AssertParameterMiddleware.php

@@ -15,7 +15,6 @@
 use TheCodingMachine\GraphQLite\Mappers\Parameters\ParameterMiddlewareInterface;
 use TheCodingMachine\GraphQLite\Parameters\InputTypeParameterInterface;
 use TheCodingMachine\GraphQLite\Parameters\ParameterInterface;
-use TheCodingMachine\Graphqlite\Validator\Annotations\Assert;
 use TheCodingMachine\Graphqlite\Validator\Annotations\Assertion;
 use function array_map;
 use function array_merge;