[likes] Don't send liked notification for posts without plain content.

[YUCLing]

Fixes #0000

Changes proposed in this pull request:

Don't send liked notifications for posts that don't have plain content.

In this approach, the extensibility of the Likes extension won't be reduced. Since it allows all other post types to be liked, it also keeps the original notification ability from the Likes extension for all other post types that contain valid plain content.

Reviewers should focus on:

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)

[luceos]

So the issue here is:

Liking any other Post type than CommentPost is causing issues with notifications.

The PR #4075 relates to this exact issue as well.

What I think should happen is create an extensible layer by adding a property to the implementation of every Post type, eg CommentPost that indicates whether notifications should be sent.

I'm closing this PR and the other one and will create an issue to explain further.