Error spam from StringLengthValidator and failing load in some feeds

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### Summary

Errors being spammed to logs when descriptions are null on images

### Steps to reproduce

Upload image with no description

Check https://www.minds.com/groups/profile/844220536770330624/feed on prod

### Platform information 

Engine

### What is the current *bug* behavior?

Error spam

### What is the expected *correct* behavior?

Should default to an empty string.

### Relevant logs and/or screenshots

```
NOTICE: PHP message: Minds.CRITICAL: TypeError: Minds\Helpers\StringLengthValidators\AbstractLengthValidator::validateMaxAndTrim(): Argument #1 ($target) must be of type string, null given, called in /var/www/Minds/engine/Entities/Image.php on line 375 and defined in /var/www/Minds/engine/Helpers/StringLengthValidators/AbstractLengthValidator.php:74 Stack trace: #0 /var/www/Minds/engine/Entities/Image.php(375): Minds\Helpers\StringLengthValidators\AbstractLengthValidator->validateMaxAndTrim(NULL) #1 /var/www/Minds/engine/Core/Notifications/Notification.php(218): Minds\Entities\Image->export() #2 /var/www/Minds/engine/Api/Exportable.php(104): Minds\Core\Notifications\Notification->export() #3 /var/www/Minds/engine/Core/Notifications/Controller.php(146): Minds\Api\Exportable->export() #4 [internal function]: Minds\Core\Notifications\Controller->getList(Object(Zend\Diactoros\ServerRequest)) #5 /var/www/Minds/engine/Core/Router/Middleware/Kernel/RegistryEntryMiddleware.php(60): call_user_func(Array, Object(Zend\Diactoros\ServerRequest)) #6 /var/www/Minds/engine/Core/Router/Dispatcher.php(57): Minds\Core\Router\Middleware\Kernel\RegistryEntryMiddleware->process(Object(Zend\Diactoros\ServerRequest), Object(Minds\Core\Router\Dispatcher)) #7 /var/www/Minds/engine/Core/Router/Middleware/LoggedInMiddleware.php(61): Minds\Core\Router\Dispatcher->handle(Object(Zend\Diactoros\ServerRequest)) #8 /var/www/Minds/engine/Core/Router/Dispatcher.php(57): Minds\Core\Router\Middleware\LoggedInMiddleware->process(Object(Zend\Diactoros\ServerRequest), Object(Minds\Core\Router\Dispatcher)) #9 /var/www/Minds/engine/Core/Router/Middleware/Kernel/RequestHandlerMiddleware.php(82): Minds\Core\Router\Dispatcher->handle(Object(Zend\Diactoros\ServerRequest)) #10 /var/www/Minds/engine/Core/Router/Dispatcher.php(57): Minds\Core\Router\Middleware\Kernel\RequestHandlerMiddleware->process(Object(Zend\Diactoros\ServerRequest), Object(Minds\Core\Router\Dispatcher)) #11 /var/www/Minds/engine/Core/Router/Middleware/Kernel/XsrfCookieMiddleware.php(32): Minds\Core\Router\Dispatcher->handle(Object(Zend\Diactoros\ServerRequest)) #12 /var/www/Minds/engine/Core/Router/Dispatcher.php(57): Minds\Core\Router\Middleware\Kernel\XsrfCookieMiddleware->process(Object(Zend\Diactoros\ServerRequest), Object(Minds\Core\Router\Dispatcher)) #13 /var/www/Minds/engine/Core/Router/Middleware/Kernel/OauthMiddleware.php(50): Minds\Core\Router\Dispatcher->handle(Object(Zend\Diactoros\ServerRequest)) #14 /var/www/Minds/engine/Core/Router/Dispatcher.php(57): Minds\Core\Router\Middleware\Kernel\OauthMiddleware->process(Object(Zend\Diactoros\ServerRequest), Object(Minds\Core\Router\Dispatcher)) #15 /var/www/Minds/engine/Core/Router/Middleware/Kernel/SessionMiddleware.php(63): Minds\Core\Router\Dispatcher->handle(Object(Zend\Diactoros\ServerRequest)) #16 /var/www/Minds/engine/Core/Router/Dispatcher.php(57): Minds\Core\Router\Middleware\Kernel\SessionMiddleware->process(Object(Zend\Diactoros\ServerRequest), Object(Minds\Core\Router\Dispatcher)) #17 /var/www/Minds/engine/Core/Router/Middleware/Kernel/FrameSecurityMiddleware.php(29): Minds\Core\Router\Dispatcher->handle(Object(Zend\Diactoros\ServerRequest)) #18 /var/www/Minds/engine/Core/Router/Dispatcher.php(57): Minds\Core\Router\Middleware\Kernel\FrameSecurityMiddleware->process(Object(Zend\Diactoros\ServerRequest), Object(Minds\Core\Router\Dispatcher)) #19 /var/www/Minds/engine/Core/Router/Middleware/Kernel/JsonPayloadMiddleware.php(40): Minds\Core\Router\Dispatcher->handle(Object(Zend\Diactoros\ServerRequest)) #20 /var/www/Minds/engine/Core/Router/Dispatcher.php(57): Minds\Core\Router\Middleware\Kernel\JsonPayloadMiddleware->process(Object(Zend\Diactoros\ServerRequest), Object(Minds\Core\Router\Dispatcher)) #21 /var/www/Minds/engine/Core/Router/Middleware/Kernel/CorsMiddleware.php(59): Minds\Core\Router\Dispatcher->handle(Object(Zend\Diactoros\ServerRequest)) #22 /var/www/Minds/engine/Core/Router/Dispatcher.php(57): Minds\Core\Router\Middleware\Kernel\CorsMiddleware->process(Object(Zend\Diactoros\ServerRequest), Object(Minds\Core\Router\Dispatcher)) #23 /var/www/Minds/engine/Core/Router/Middleware/Kernel/RouteResolverMiddleware.php(68): Minds\Core\Router\Dispatcher->handle(Object(Zend\Diactoros\ServerRequest)) #24 /var/www/Minds/engine/Core/Router/Dispatcher.php(57): Minds\Core\Router\Middleware\Kernel\RouteResolverMiddleware->process(Object(Zend\Diactoros\ServerRequest), Object(Minds\Core\Router\Dispatcher)) #25 /var/www/Minds/engine/Core/Router/Middleware/Kernel/ErrorHandlerMiddleware.php(64): Minds\Core\Router\Dispatcher->handle(Object(Zend\Diactoros\ServerRequest)) #26 /var/www/Minds/engine/Core/Router/Dispatcher.php(57): Minds\Core\Router\Middleware\Kernel\ErrorHandlerMiddleware->process(Object(Zend\Diactoros\ServerRequest), Object(Minds\Core\Router\Dispatcher)) #27 /var/www/Minds/engine/Core/Router/Middleware/Kernel/ContentNegotiationMiddleware.php(45): Minds\Core\Router\Dispatcher->handle(Object(Zend\Diactoros\ServerRequest)) #28 /var/www/Minds/engine/Core/Router/Dispatcher.php(57): Minds\Core\Router\Middleware\Kernel\ContentNegotiationMiddleware->process(Object(Zend\Diactoros\ServerRequest), Object(Minds\Core\Router\Dispatcher)) #29 /var/www/Minds/engine/Core/Router.php(105): Minds\Core\Router\Dispatcher->handle(Object(Zend\Diactoros\ServerRequest)) #30 /var/www/Minds/engine/index.php(9): Minds\Core\Router->route() #31 {main}         {"exception":"[object] (TypeError(code: 0): Minds\\Helpers\\StringLengthValidators\\AbstractLengthValidator::validateMaxAndTrim(): Argument #1 ($target) must be of type string, null given, called in /var/www/Minds/engine/Entities/Image.php on line 375 at /var/www/Minds/engine/Helpers/StringLengthValidators/AbstractLengthValidator.php:74)"}
```

### Possible fixes

(If you can, link to the line of code that might be responsible for the problem)

Story points: 1.0