Fixes #3838

Changes proposed in this pull request:

• Only set opacity of Scrubber-unread to 1 if unreadPercent is greater then 0.

Reviewers should focus on:
As discovered in #3838 and verified just now, unreadPercent evaluates to a negative value when composing a new post. This is because this.stream.index returns unexpected values.

Ideally, the issue would be fixed at its root cause, but for backwards compatibility I feel like it's better if the PostStreamState is left untouched and the issue be fixed in the styleUnread function itself

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)