getKeyValue() fatals with TypeError when the private key file is missing (dirname(false))

Problem/Motivation

SitePrivateKeyFileKeyProviderDecorator::getKeyValue() calls dirname($this->fileSystem->realpath($path)). realpath() returns FALSE when the private key file is missing, so dirname(false) throws a TypeError on PHP 8. Any page running easy_encryption_requirements() (e.g. /admin/config) then fatals.

TypeError: dirname(): Argument #1 ($path) must be of type string, false given (line 67 of SitePrivateKeyFileKeyProviderDecorator.php)

A second TypeError follows: parent::getKeyValue() can return NULL against the : string return type.

Steps to reproduce

1. Have an easy_encrypted key whose private key file is missing on this environment.
2. Visit /admin/config.
3. WSOD / TypeError.

Not a duplicate of #3575553

#3575553 (fixed, 1.0.0-rc6, in 1.0.3) fixed key generation: writePrivateKeyToFile() now creates the directory. This is the read path — getKeyValue() still calls dirname(realpath()) unguarded in 1.0.3.

Proposed resolution

Guard realpath() !== FALSE before dirname(), and return ?? '' from the parent delegations. MR to follow.

AI usage

AI-Generated: Yes (Used Claude Code to draft the issue summary and the proposed fix; reviewed and verified by the contributor.)