If a configuration file is passed with a command line argument and the file is not found, Radicale shows a warning and continues with the default configuration.
There is no reason for doing this, Radicale should just fail.
Instead, this PR allows passing an empty string like ``--config ""``. Radicale will use the default configuration in this case, without trying to load the configuration from the common paths. Previously you had to do specify a path that doesn't exist like ``--config /does/not/exist``, which looks a bit ugly and showed a warning message.
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