Fix problems reported by PHPStan

[szepeviktor]

It would be nice to have a UsesXPathFilters::filterExtractAttribute beside filterExtractAttributes.
One would return array<string>, the other array<array<string>>.

[spekulatius]

Which use-cases do you want to cover? You could also try filterFirstExtractAttribute to see if it works for your needs.

[szepeviktor]

Which use-cases do you want to cover?

To make static analysis possible.
Because the return types are different.

I know, I know. Popular software come with the liberty of using one method for several purposes. But I am not intended to be popular!

[spekulatius]

Okay, feel free to propose a PR :) Maybe after we got the PHPStan branch merged?

[szepeviktor]

I do not have plans to contribute more.

[spekulatius]

Okay