[2.x] fix: allow boolean settings with truthy defaults to be disabled

[huoxin233]

Fixes #4781

Changes proposed in this pull request:

Resolves a bug in AdminPage.tsx where boolean settings (toggles) that have a truthy default value could not be turned off by the administrator.

By switching from the logical OR operator (||) to the nullish coalescing operator (??), the setting stream correctly recognizes "" as a valid saved state. The fallback is now only applied when the setting is strictly undefined or null (e.g., during a fresh installation where the setting hasn't been saved to the database yet).

Reviewers should focus on:

Screenshot

PixPin_2026-06-26_18-10-57.mp4

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)