Provide correct defaults for CoinBase options flow

[TomBrien]

Proposed change

In absence of any preconfigured values (i.e. at first run) the form was trying to set None as the default for the multi_selects. This caused a malformed data error, this PR fixes the defaults to empty lists.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
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• Tests have been added to verify that the new code works.

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