Fix PHP Fatal error when SyncProgress started_at is not set

- PHP Warning: Trying to access array offset on value of type null
- PHP Fatal error: Uncaught TypeError: MedusaWP\Models\SyncProgress::count_troubleshoot_messages(): Argument #1 ($sync_timestamp) must be of type int, null given

wordpress-plugin/src/Routes/Admin/MedusaBulkSync.php

@@ -668,7 +668,7 @@ public function get_sync_progress() {
 	 */
 	public function get_sync_progress_messages( $req ) {
 		$progress       = Settings::get_sync_progress();
-		$sync_timestamp = $progress['started_at'];
+		$sync_timestamp = isset( $progress['started_at'] ) ? $progress['started_at'] : time();
 
 		$params = $req->get_params();