[BUGFIX] Fix addService PHP 8 warning

If a service is registered with ExtensionManagementUtility::addService(),
that does not have a "os" flag, then a PHP 8 warning
is raised (during loading).

This change avoids this warning.

Resolves: #99294
Releases: main, 11.5
Change-Id: Ie5427e1a48f32dccbf1c5618a199c9a4d55774d3
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/76942


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>