fix php 8.1 Deprecated: htmlspecialchars(): Passing null to parameter #1

 (#1570)

* Fix multiple calls returning the same object

* fix: strtolower() passing null to parameter #1 ($string) of type string is deprecated

* fix: php 8.1 Deprecated: htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated

var/Typecho/Widget/Helper/Form/Element.php

@@ -217,7 +217,7 @@ abstract public function input(?string $name = null, ?array $options = null): ?L
     public function value($value): Element
     {
         $this->value = $value;
-        $this->inputValue($value);
+        $this->inputValue($value ?? '');
         return $this;
     }