Make changes discussed in https://github.com/AlekSi/nut/issues/28

> 1. .nut.json should not be created if it's not present. Use-case: invocation of nut get on server.
no changes required

> 2. .nut.json is updated by any invocation of nut command.
no changes required

> 3. If .nut.json is empty, it should be generated silently, without "unexpected end of JSON input" warning. Use-case: first invocation.
done

> 4. If .nut.json is not empty and incorrect, do not update it and exit with error message.
done