apt update: Can not add an empty (zero-length) key to the cache

This string does not exist in apt or any of the other Ubuntu code I have checked out. If you can reproduce it, do check where it is coming from and reassign it there.

It is coming from the appstream source package.

Steps to reproduce:
1. Create a minimal Ubuntu 20.04 chroot with all packages that install files in /etc/apt/apt.conf.d on my Ubuntu 20.04 VM installed (that is, update-notifier-common, debconf, aptdaemon, apt-config-icons, apt-config-icons-hidpi, packagekit, appstream, apt, unattended-upgrades, ubuntu-advantage-tools, snapd and command-not-found).
2. Get the source code for all packages in the chroot with apt source SOURCE_PACKAGE_NAMES.
3. Run the following command:

$ grep -Fr 'Can not add an empty (zero-length) key to the cache'
appstream-0.12.10/.pc/01_less-long-token-noise.patch/src/as-cache.c: "Can not add an empty (zero-length) key to the cache");
appstream-0.12.10/src/as-cache.c: "Can not add an empty (zero-length) key to the cache");

This issue does not longer exist in appstream since jammy - so the quickest fix is to simply upgrade to that release.

This just happened again.