Fix compatibility with PHP 8.1 (#1324)

* Fix compatibility with PHP 8.1

PHP Deprecated:  htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated in typecho/var/Widget/Archive.php on line 1292

* Fix login

* Fix write

* Fix manage

* Fix PHP Deprecated

admin/login.php

@@ -4,7 +4,7 @@ include 'common.php';
 if ($user->hasLogin()) {
     $response->redirect($options->adminUrl);
 }
-$rememberName = htmlspecialchars(\Typecho\Cookie::get('__typecho_remember_name'));
+$rememberName = htmlspecialchars(\Typecho\Cookie::get('__typecho_remember_name', ''));
 \Typecho\Cookie::delete('__typecho_remember_name');
 
 $bodyClass = 'body-100';
@@ -25,7 +25,7 @@ include 'header.php';
             </p>
             <p class="submit">
                 <button type="submit" class="btn btn-l w-100 primary"><?php _e('登录'); ?></button>
-                <input type="hidden" name="referer" value="<?php echo htmlspecialchars($request->get('referer')); ?>" />
+                <input type="hidden" name="referer" value="<?php echo htmlspecialchars($request->get('referer') ?? ''); ?>" />
             </p>
             <p>
                 <label for="remember">