Handle empty message queue in CachePoolStore::pop()

Author: MartkCz

src/Server/Transport/Sse/Store/CachePoolStore.php

@@ -45,6 +45,11 @@ public function pop(Uuid $id): ?string
         }
 
         $messages = $item->get();
+
+        if ([] === $messages) {
+            return null;
+        }
+
         $message = array_shift($messages);
 
         $item->set($messages);