core: 修复ua无法识别时打开home_security错误的问题

packages/hydrooj/package.json

@@ -1,6 +1,6 @@
 {
     "name": "hydrooj",
-    "version": "2.19.0",
+    "version": "2.19.1",
     "bin": "bin/hydrooj.js",
     "main": "dist/loader.js",
     "typings": "dist/loader.d.ts",

packages/hydrooj/src/lib/useragent.ts

@@ -4,7 +4,7 @@ export function parse(str: string) {
     return browser.parseUserAgent(str);
 }
 
-export function icon(str: string) {
+export function icon(str: string = '') {
     return str.split(' ')[0].toLowerCase();
 }