fix nullable parameter error

var/Typecho/Widget/Request.php

@@ -245,7 +245,7 @@ class Request
      * @param string|null $default
      * @return string|null
      */
-    public function getServer(string $name, string $default = null): ?string
+    public function getServer(string $name, ?string $default = null): ?string
     {
         return $this->request->getServer($name, $default);
     }

var/Typecho/Widget/Response.php

@@ -184,7 +184,7 @@ class Response
      * @param string|null $suffix 附加地址
      * @param string|null $default 默认来路
      */
-    public function goBack(string $suffix = null, string $default = null)
+    public function goBack(?string $suffix = null, ?string $default = null)
     {
         //获取来源
         $referer = $this->request->getReferer();