Fixes compatibility two polyfill functions with their behavior in mbsting extension in php 8.0

[r00d1k]

I tried to use symfony/console in my project. I use php 8.0 without mbstring ext. When console script runs without arguments I get this two fatals
PHP Fatal error: Uncaught TypeError: mb_detect_encoding(): Argument #1 ($string) must be of type string, null given
PHP Fatal error: Uncaught TypeError: mb_strwidth(): Argument #1 ($string) must be of type string, null given

In mbstring ext these functions returns string (5) 'ASCII' if null passed.

[symfony-bot]

Thanks for your pull request! We love contributions.

However, this repository is what we call a "subtree split": a read-only copy of one directory of the main Symfony polyfills repository. It is used by Composer to allow developers to depend on specific Symfony PHP polyfills.

If you want to contribute, you should instead open a pull request on the main repository:

https://github.com/symfony/polyfill

Thank you for your contribution!

PS: if you haven't already, please add tests, and beware that bug fixes should be submitted on the lowest maintained branch where they apply; only features should be submitted against the main branch.

[nicolas-grekas]

Will be fixed by symfony/polyfill#330