Empty pockets don't end up in the cache - E: The value 'hirsute-backports' is invalid for APT::Default-Release as such a release is not available in the sources

[Julian Andres Klode]

[Impact]
If a pocket is empty, apt will not download the empty packages files, causing the repository to not be added to the cache. This in turn breaks commands such as using apt install -t hirsute-backports as apt is unable to find the release in the archive.

[Test plan]
Assuming an empty pocket is around, use apt install -t <release>-<pocket> to see it's in the cache.

An appropriate test case for empty stuff should be added to the test suite as well (FIXME before SRUing), the test suite is extensive and prevents most regressions

[Where problems could occur]
TBD

[Other info]
Originally reported at https://www.reddit.com/r/Kubuntu/comments/n9vg9e/cant_make_backports_repo_for_hirsute_to_work/