fix php 8.1 error when autoFilter['ref'] is null in AutoFilter

[brainfoolong]

This is:

- [x] a bugfix

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

On PHP 8.1, when loading an existing file into PhpSpreadsheet, this error does happen sometimes. This change will fix this issue. I cannot provide a testfile, as this happen only on very specific files that i cannot post here, for data protection reasons. I tried creating a reproducible file for here, but it didn't worked out.

I don't know why autoFilter['ref'] is null, but it obviously is sometimes null. The key exists for sure, but it's null. Maybe it can cause other problems, but this fixed it for us.