_frontend/cli.py: Fix `artifact checkout` with no arguments

When no arguments are passed to artifact checkout command, it currently crashes. We actually have a check of this, but we crash before that becasue this method tries to read the value of target before we've had time to check for it.

That can only be done correctly after the app has been initialized. So, refactor that bit of the method to run after we've checked that we are working with a non-empty target.

Also, add a regression test for it.

Fixes #1367 (closed).