Revert "argparse: Empty args are empty"

This reverts commit 658cf07fdeb743a6f78ba8d59efb60116fe473e2.

This breaks `ostbuild make`.
builtin: Don't emit a useless stack trace when a builtin fails
builtin: Remove some unnecessary variables from builtins
Use libostree to start doing simple operations

Instead of forking out to a subprocess.