Fix PHP 8.4 Implicitly marking parameter $previous as nullable is deprecated

[BrookeDot]

What?

PHP 8.4 requires null values to be explicitly declared: https://www.php.net/manual/en/migration84.deprecated.php

This PR does that for the U2F.php class fixing the warning:

u2flib_server\Error::__construct(): Implicitly marking parameter $previous as nullable is deprecated, the explicit nullable type must be used instead

Why?

To avoid noise in the PHP logs and it is likely this may be a fatal in a future version of PHP.

How?

This adds a ? to the class to allow it to be explicitly null.

Testing Instructions

Install PHP 8.4, make the change in the PR and watch the error go away.

Changelog Entry

Fixed - PHP 8.4: Implicitly marking parameter $previous as nullable is deprecated (#663)

[kasparsd]

The plugin is currently supporting PHP 7.2 and above:

https://github.com/WordPress/two-factor/blob/953b7c98a51d3ab2f3d2c6c25f007f626be4b7de/two-factor.php#L16C23-L16C26

It appears that support for nullable arguments was added to 7.1 so this is safe to merge.

Thanks for the pull request!

Noting that U2F support will be removed soon #439 but having this fixed is definitely worth it.