fix: initialize compatibility attestation in guest-agent test

Author: kvinwang

dstack/guest-agent/src/rpc_service.rs

@@ -897,6 +897,7 @@ pNs85uhOZE8z2jr8Pg==
                     .unwrap_or_default(),
                     report_data: report_data.to_vec(),
                     vm_config: vm_config.to_string(),
+                    attestation: Vec::new(),
                     event_log_ccel: Vec::new(),
                 })
             }