Change import command's config default argument

[JimMadge]

• Resolves: Support migration of user data #5235
• Target version: main

Summary

• Change the default argument of deck:import --config ... from config.json to null.
  This default value caused a problem (Support migration of user data #5235) where if not config was supplied, config.json is not found and the message "It's not a valid config file" is shown (deck:import misleading error message for config.json #5213)
• Change the deck:import --data ... option to VALUE_REQUIRED. From reading the Symfony docs I think all options are optional. VALUE_OPTIONAL lets you include the option in a command line without giving a value, e.g. deck:import --data would be valid.

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
  • Is this n/a as there are no new features?
• Documentation (manuals or wiki) has been updated or is not required
  • I think this should be consistent with the current docs, where not providing --config assumes the data is a deck json.

[juliusknorr]

Thanks for your pull request. 👍

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