Fix PHPStan command and nullsafe usage

.github/workflows/backend-quality.yml

@@ -35,7 +35,7 @@ jobs:
         run: composer install --no-interaction --prefer-dist
 
       - name: Run PHPStan
-        run: vendor/bin/phpstan analyse --memory-limit=1G
+        run: vendor/bin/phpstan analyse --memory-limit=-1
 
   coding-standards:
     name: ECS

server/src/Service/SignalSubmissionService.php

@@ -81,7 +81,7 @@ final class SignalSubmissionService
             $retryAfter = $limit->getRetryAfter();
             $this->logger->warning('Signal submission rejected due to rate limiting.', [
                 'client_key' => $clientKey,
-                'retry_after' => $retryAfter?->format(DATE_ATOM),
+                'retry_after' => $retryAfter->format(DATE_ATOM),
             ]);
             throw new SubmissionRateLimitedException($limit->getRetryAfter());
         }