Fix strip_tags in ProgramYearController

[sentry-io]

TypeError: strip_tags() expects parameter 1 to be string, null given
Sentry Issue: API-6P

TypeError: strip_tags() expects parameter 1 to be string, null given
  File "/var/www/ilios/src/Controller/ProgramYearController.php", line 131, in strip_tags
    $row[$key] = strip_tags($row[$key]);
  File "/var/www/ilios/src/Controller/ProgramYearController.php", line 131, in App\Controller\ProgramYearController::App\Controller\{closure}
    $row[$key] = strip_tags($row[$key]);
  File "/var/www/ilios/src/Controller/ProgramYearController.php", line 134, in App\Controller\ProgramYearController::downloadCourseObjectivesReportAction
    });
  File "/var/www/ilios/vendor/symfony/http-kernel/HttpKernel.php", line 146, in Symfony\Component\HttpKernel\HttpKernel::handleRaw
    $response = $controller(...$arguments);
  File "/var/www/ilios/vendor/symfony/http-kernel/HttpKernel.php", line 68, in Symfony\Component\HttpKernel\HttpKernel::handle
    return $this->handleRaw($request, $type);
...
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