Don't show excerpt in PostPreview component if there are no plain content

[the-turk]

Fixes #2942

Reviewers should focus on:
span.PostPreview-excerpt is still necessary for proper view.

Quoting from @SychO9:

Besides fixing the JS error, it would be a good idea to improve the preview for an event post mention, even though people aren't supposed to reply to event posts, they still can by manually writing the post mention, as long as they have the post's ID.

Since we don't store any information about event posts in db, I think this kind of enhancement will need some breaking changes.

Confirmed

• Frontend changes: tested on a local Flarum installation.

[SychO9]

Thank you for the PR!

This is enough, I don't think there is any value in it containing info about the event post, since such post mentions shouldn't happen anyway.

we don't store any information about event posts in db

We do btw, the posts themselves have info on the event, thus event posts.

[the-turk]

We do btw, the posts themselves have info on the event, thus event posts.

I mean, yeah, but we don't have the information on how to turn that into readable text like " x locked the discussion". All info we have is it's an event post which has 'locked' type and nothing more.

[davwheat]

Coincidence! I was just taking a look at the issue for this PR :P

Looks good to me and appears to work locally 👍