[experiment] Properly handle "default" case

There's no prototype available for the experiment as it
needs to be configured with the content of a prefix based
on the toolchain selected for it.
2 jobs for 79_implement_missing_prototypes_handling in 16 minutes and 19 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #164480
docker-build
build_linux_36

00:14:42

65.0%
passed #164481
macosx
build_macosx_36

00:16:19

65.0%