Fix null array offset in _destroy() for PHP 8.5

[kolotov]

In PHP 8.5, using null as an array offset is deprecated.
The _destroy() method accesses self::$cleaned_ids[$this->file_id]
without checking if $this->file_id is null, triggering a deprecation warning.

Add an early return when $this->file_id is null.

Fixes: #852

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[williamdes]

I am not sure this is correct, there is more code that should maybe be executed

[kolotov]

You're right. I'll think about it some more.

[renanrodrigo]

As this is just about unsetting destroyall when the file has already been cleaned, it's enough to check if there is any file_id at all. If it's null, the second part of the method can still unset the other properties if destroyall is set.

--- a/tcpdf.php
+++ b/tcpdf.php
@@ -7882,7 +7882,7 @@
 	 * @since 4.5.016 (2009-02-24)
 	 */
 	public function _destroy($destroyall=false, $preserve_objcopy=false) {
-		if (isset(self::$cleaned_ids[$this->file_id])) {
+		if (isset($this->file_id) && isset(self::$cleaned_ids[$this->file_id])) {
 			$destroyall = false;
 		}
 		if ($destroyall AND !$preserve_objcopy && isset($this->file_id)) {

[kolotov]

@renanrodrigo Good point, I have applied your suggested change.