PHP 8.1 fix deprecated null parameters (#187)

* fix #186 ($string) of type string is deprecated in PHP 8.1 * fix DOMDocument::__construct(): Passing null to parameter #2 ($encoding) of type string is deprecated in vendor/spatie/array-to-xml/src/ArrayToXml.php on line 29 * Update ArrayToXml.php

Author: gigerIT

src/ArrayToXml.php

@@ -26,7 +26,7 @@ public function __construct(
  array $domProperties = [],
  ?bool $xmlStandalone = null
  ) {
- $this->document = new DOMDocument($xmlVersion, $xmlEncoding);
+ $this->document = new DOMDocument($xmlVersion, $xmlEncoding ?? '');
 
  if (! is_null($xmlStandalone)) {
  $this->document->xmlStandalone = $xmlStandalone;
@@ -141,7 +141,7 @@ protected function convertElement(DOMElement $element, mixed $value): void
  $sequential = $this->isArrayAllKeySequential($value);
 
  if (! is_array($value)) {
- $value = htmlspecialchars($value);
+ $value = htmlspecialchars($value ?? '');
 
  $value = $this->removeControlCharacters($value);