Fix deprecation and undefined array key

[froboy]

This resolves two issues:

1. When loading get-pdf, a "creation of dynamic property" warning.

   Deprecated function: Creation of dynamic property Drupal\openy_repeat\OpenyRepeatRepository::$memoryCache is deprecated in Drupal\openy_repeat\OpenyRepeatRepository->__construct() (line 62 of /var/www/html/docroot/modules/contrib/openy_repeat/src/OpenyRepeatRepository.php)

2. When loading an empty PDF, an "undefined array" warning:

   Warning: Undefined array key "content" in Drupal\openy_repeat\Controller\RepeatController->getPdf() (line 549 of /var/www/html/docroot/modules/contrib/openy_repeat/src/Controller/RepeatController.php)

[froboy]

Oops! Looks like this duplicates #68.