Show a feed that answers with nothing, having never posted, as failing (#119)

daily-quests and hots-dots had 0 items and no error, so their subscribers saw shows that had not
posted yet rather than feeds that are gone. Both domains have lapsed, and this network's DNS
filter answers for a parked domain with its block page, over http a 200 with an empty body.
scan_one took a body of nothing but whitespace for "nothing yet" and recorded no error.

It still does for a feed that has stored items, which is what the Antarctic Survey's empty 202
means, but a feed that has never stored one now fails with "the feed's address answered with
nothing at all", which the feed's page explains as an address that answers with nothing, the
site maybe gone. The next real read clears it. A test serves both from a socket of its own.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- A feed whose address answers with nothing, as a lapsed domain does behind a DNS filter's block page, shows as failing, with what to do about it, instead of as a show that has not posted yet.
 - On Postgres, a write no longer waits for the database server's disk. A scan read about one feed a second and now reads 1,500 in 20 seconds.
 - Checking every feed no longer makes every icon in the feed list flash while it runs: the list keeps the icons it has already drawn.