(bug): error produced when reporting group for any reason

Groups cannot be reported, there is an error.

:update @benhayward.ben

Request is 500ing throwing this into the nginx and php-fpm containers.

nginx_1          | 2019/09/12 17:07:53 [error] 6#6: *3362 FastCGI sent in stderr: "PHP message: Exception #1568308073: Cassandra\Exception\InvalidArgumentException: value must be a long, a double, a numeric string or a Cassandra\Bigint,  given in /var/www/Minds/engine/Core/Reports/UserReports/Repository.php:60
nginx_1          | Stack trace:
nginx_1          | #0 /var/www/Minds/engine/Core/Reports/UserReports/Repository.php(60): Cassandra\Bigint->__construct(NULL)
nginx_1          | #1 /var/www/Minds/engine/Core/Reports/UserReports/Manager.php(96): Minds\Core\Reports\UserReports\Repository->add(Object(Minds\Core\Reports\UserReports\UserReport))
nginx_1          | #2 /var/www/Minds/engine/Controllers/api/v2/moderation/report.php(77): Minds\Core\Reports\UserReports\Manager->add(Object(Minds\Core\Reports\UserReports\UserReport))
nginx_1          | #3 /var/www/Minds/engine/Api/Factory.php(69): Minds\Controllers\api\v2\moderation\report->post(Array)
nginx_1          | #4 /var/www/Minds/engine/Controllers/api/api.php(81): Minds\Api\Factory::build(Array, Object(Zend\Diactoros\ServerRequest), Object(Zend\Diactoros\Response\JsonResponse))
nginx_1          | #5 /var/www/Minds/engine/Core/Router.php(137): Minds\Controllers\api\" while reading response header from upstream, client: 172.19.0.1, server: _, request: "POST /api/v2/moderation/report HTTP/1.1", upstream: "fastcgi://172.19.0.8:9000", host: "0.0.0.0", referrer: "http://0.0.0.0/groups/profile/1018930846406545413/feed"
elasticsearch_1  | [2019-09-12T17:08:08,994][INFO ][o.e.c.r.a.DiskThresholdMonitor] [HhjXpE-] low disk watermark [85%] exceeded on [Hh