swh scanner fail to start when configuration file is missing

$ swh scanner scan --help
Usage: swh scanner [OPTIONS] COMMAND [ARGS]...
Try 'swh scanner -h' for help.

Error: Invalid value for '-C' / '--config-file': File '/home/zack/.config/swh/global.yml' does not exist.

Looks like a regression introduced when improving configuration file handling. The desired behavior is that swh scanner scan works even without a configuration file (as a general principle: configuration files should not be mandatory for simple CLI tools).

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Migrated from T2632 (view on Phabricator)