Initialize $permissions variable in ActivityPub\Transmitter::fetchPermissionBlockFromConversation

- Address https://github.com/friendica/friendica/issues/10757#issuecomment-927361800

src/Protocol/ActivityPub/Transmitter.php

@@ -437,6 +437,13 @@ class Transmitter
 			return [];
 		}
 
+		$permissions = [
+			'to' => [],
+			'cc' => [],
+			'bto' => [],
+			'bcc' => [],
+		];
+
 		$activity = json_decode($conversation['source'], true);
 
 		$actor = JsonLD::fetchElement($activity, 'actor', 'id');