luigid: allow empty state-path and logdir for development

Since d54a15a2e1453c36a6470d08efd01e23d0b26233 it was not possible to run `luigid` from the command line without specifying a `state_path` and a `log_dir`. This PR fixes this (under the assumption, that running `luigid` in non-deamon mode should *by default* log to stderr and should not require a `state_path`).