fix: 下载配置接口加null检查，前端去掉responseType:text让axios自动解析

par-api/src/main/java/com/par/api/controller/ConfigController.java

@@ -80,6 +80,9 @@ public class ConfigController {
             log.error("Failed to read config: siteId={}, version={}", siteId, version, e);
             return ResponseEntity.status(500).body("Failed to read config");
         }
+        if (content == null) {
+            return ResponseEntity.notFound().build();
+        }
 
         return ResponseEntity.ok()
                 .eTag(config.getConfigHash())