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.