Guard against empty cache event keys

Author: cleptric

src/Sentry/Laravel/Features/CacheIntegration.php

@@ -87,7 +87,10 @@ public function handleCacheEventsForBreadcrumbs(Events\CacheEvent $event): void
                 return;
         }
 
-        $displayKey = $this->replaceSessionKey($event->key);
+        $displayKey = '';
+        if ($event->key !== null) {
+            $displayKey = $this->replaceSessionKey($event->key);
+        }
 
         Integration::addBreadcrumb(new Breadcrumb(
             Breadcrumb::LEVEL_INFO,