Ouzo\Restrictions::isNull PHP Method

    public static function isNull()
    {
        return new IsNullRestriction();
    }

 /**
  * @test
  */
 public function shouldCreateSql()
 {
     //given
     $restriction = Restrictions::isNull();
     //when
     $sql = $restriction->toSql('category_id');
     //then
     $this->assertEquals('category_id IS NULL', $sql);
 }