fix(core): change condition when unread label is shown in Scrubber

[DavideIadeluca]

2.x port of #4116

Progresses #4060

Changes proposed in this pull request:
Change condition when unread label is shown in Scrubber, prevents 0 UNREAD being shown.

Reviewers should focus on:
See #4116 and #3838 for more details

Screenshot

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)