Polish `Details` panel; added "Not Applicable" to fill otherwise blank spaces

Summary: Aligned detail item labels; added "Not Applicable" when PolicyKit info is missing

Test Plan:
- Apply patch
- Kill and relaunch `polkit-kde-authentication-agent-1`
- Launch an application or change a KCM that prompts for authorization
- Check that details are no longer left blank

Reviewers: davidedmundson, ngraham, #plasma

Reviewed By: davidedmundson, ngraham, #plasma

Subscribers: abetts, plasma-devel, #plasma

Tags: #plasma

Maniphest Tasks: T8569

Differential Revision: https://phabricator.kde.org/D12479