User config build workflow fails when build matrix is empty #3179
Description
When you create a user config repo using https://github.com/zmkfirmware/unified-zmk-config-template as a template, the first thing you see is your new repo. The second is a failed build action.
The build matrix we generate from an empty build.yaml is an empty string, so the build fails with an error like:
Build ZMK firmware
Error when evaluating 'strategy' for job 'build'. zmkfirmware/zmk/.github/workflows/[email protected] (Line: 50, Col: 15): Unexpected value ''
When the build matrix is empty, we should bail and mark the build as skipped (if possible, or successful otherwise) along with an annotation explaining that nothing was built, ideally with a link to https://zmk.dev/docs/user-setup#add-a-keyboard or other documentation on how to add a keyboard to the build.