Fix null array offset in _destroy() for PHP 8.5

Author: kolotov

tcpdf.php

@@ -7884,6 +7884,9 @@ public function Output($name='doc.pdf', $dest='I') {
 	 * @since 4.5.016 (2009-02-24)
 	 */
 	public function _destroy($destroyall=false, $preserve_objcopy=false) {
+		if ($this->file_id === null) {
+			return;
+		}
 		if (isset(self::$cleaned_ids[$this->file_id])) {
 			$destroyall = false;
 		}