fix: notification subject discussion eager loading fails

[SychO9]

Closes #3778

Changes proposed in this pull request:
In 1.7 the property_exists check was added to fix PHP 8.2 deprecations but missed the fact that dynamic properties cannot be checked like that.

I don't believe this is the problem described in the issue however, only 10 notifications are loaded at a time. So it could be a bad extension.

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.