[2.x] fix: distinguish a missing announcement excerpt from an empty one

[imorland]

The announcements fetcher read each excerpt from the firstPost include and ran it through makeExcerpt() whether or not that post arrived:

'excerpt' => $this->makeExcerpt(Arr::get($firstPost, 'attributes.contentHtml', '')),

With no post, Arr::get(null, …) yields '', so the excerpt became an empty string — indistinguishable from a post that genuinely has no text.

That is how the admin announcements widget came to render every card blank without anyone noticing. The response looked well-formed and said, in effect, "these announcements have no content". The real fault was upstream: fof/gamification had narrowed the firstPost eager load on the discussion index, so discuss.flarum.org stopped serializing the include at all (fixed in FriendsOfFlarum/gamification#169).

Since every Flarum forum pulls this endpoint, that single upstream change blanked the widget everywhere.

The change

excerpt is now null where the post never arrived, and '' only where the post is genuinely empty.

Nothing renders differently — AnnouncementItem already guards with {a.excerpt && …} — but a recurrence becomes legible rather than silent. The AnnouncementData type is updated to match, since it can now be null.

Tests

Three added. The existing ten all hand-build their own included array, so none of them could notice the request losing its include, nor the include coming back empty — which is exactly the gap this fell through.

• the request asks for the relationships the excerpt and author need — asserts include contains firstPost and user, via a Guzzle history middleware. Passes today; would have caught a lost include.
• a missing include yields null — fails without this change (Failed asserting that '' is null). I checked by reverting it.
• an empty post still yields an empty string — so the distinction holds in both directions and this isn't just "null everywhere".

Core unit suite green: 405 tests. Also drops a deprecated ReflectionProperty::setAccessible() call flagged on PHP 8.5, taking the suite's deprecation count from 2 to 1.