Throw exception in \Drupal\Core\StreamWrapper\PrivateStream::getDirectoryPath() when basepath() is null

Problem/Motivation

Follow-up from #3239761-16: Fix MimeTypeTest to prevent deprecations in PHP 8.1 and fix \Drupal\Core\StreamWrapper\PrivateStream::basePath() documentation →

Steps to reproduce

The \Drupal\Core\StreamWrapper\PrivateStream::getDirectoryPath() could get NULL from basepath() method as #3239761: Fix MimeTypeTest to prevent deprecations in PHP 8.1 and fix \Drupal\Core\StreamWrapper\PrivateStream::basePath() documentation → documents

Proposed resolution

throw exception to prevent access to / (root file system) when accessing non configured private files

Remaining tasks

review, patch, commit

User interface changes

tbd

API changes

tbd

Data model changes

no

Release notes snippet

tbd