CLI throws an error if there is no detections dir

[timoguin]

Problem

If the Matano CLI doesn't find a detections/ dir, it throws an error:

$ matano diff --user-directory ./config/envs/my-env
 ›   Error: ENOENT: no such file or directory, scandir '/my-homedir/my-repo-path/config/envs/my-env/detections'
exit status 1

Solution

If there is no detections/ dir, the CLI should no-op instead of throwing an error.