test: cover empty select branches reported as uncovered by Coveralls
Add tests exercising the two branches:
- httpserver: the default branch of the non-blocking serve failure publish, taken when the serveErr buffer is already full
- valkey: the subscription context Done branch in the delivery callback, taken when Close tears down the subscription while a message is blocked waiting for a consumer
Full Changelog: v1.147.0...v1.147.1