fix: 兼容空的订阅链接

backend/src/utils/download.js

@@ -15,7 +15,7 @@ import $ from '@/core/app';
 const tasks = new Map();
 
 export default async function download(
-    rawUrl,
+    rawUrl = '',
     ua,
     timeout,
     proxy,

backend/src/utils/flow.js

@@ -11,7 +11,7 @@ export function getFlowField(headers) {
     return headers[subkey];
 }
 export async function getFlowHeaders(rawUrl, ua, timeout, proxy, flowUrl) {
-    let url = flowUrl || rawUrl;
+    let url = flowUrl || rawUrl || '';
     let $arguments = {};
     const rawArgs = url.split('#');
     url = url.split('#')[0];