Set a default for add-on options and schema config

[frenck]

Proposed change

It is always nice if an add-on doesn't need any options (and thus might not have a schema).

This PR adds a default empty dictionaries to the schema and options configuration parameters of an add-on configuration, mainly for cleaner add-on configurations.

For a future improvement, it would be nice if we could expose if the add-on has options (and thus a schema) and hide the config from the UI based on that.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (which adds functionality to the supervisor)
• 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: Set a default for add-on options and schema config developers.home-assistant#771

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast supervisor tests)
• Tests have been added to verify that the new code works.

If API endpoints of add-on configuration are added/changed:

• Documentation added/updated for developers.home-assistant.io