fix(likes): don't let a liked post with no text break the notification list

[imorland]

Refs #4108

Changes proposed in this pull request:
The API accepts a like on any post type, but only comment posts have text. When someone likes, say, a "discussion renamed" post, the author's liked notification throws while building its excerpt, and the author's whole notification list renders nothing, other notifications included. The excerpt now falls back to an empty string, as the mentions notification already does. #4108 stays open for the extensible "can this post type notify" design.

Reviewers should focus on:

• Likes on non-comment posts are still accepted: restricting them was turned down in [1.x] [likes] fix: Ignore isLiked when post is not comment post. #4075 and [likes] Don't send liked notification for posts without plain content. #4095 for limiting extensions.

Screenshot
N/A

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.
• Frontend changes: tests are green (run yarn test in js/).
• Frontend changes: tests have been added, or are not appropriate here.
• Backend changes: tests are green (run composer test). (No backend changes.)
• Backend changes: tests have been added, or are not appropriate here. (No backend changes.)
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite). (No database changes.)
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.