Fix Slack "invalid_blocks_format" bug

[bachya]

Proposed change

Through a (presumably recent) API change, Slack no longer allows an empty dict to be passed to the blocks parameter. This PR fixes that issue. Also, while in there, it reduces some unnecessary code and adds me as the codeowner for the integration (thought I did that previously).

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

Example entry for configuration.yaml:

# Example configuration.yaml

Additional information

• This PR fixes or closes issue: fixes Slack throws error with simple message #43874
• This PR is related to issue: N/A
• Link to documentation pull request: N/A

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
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• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

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  Updated and included derived files by running: python3 -m script.hassfest.
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  Updated by running python3 -m script.gen_requirements_all.
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[dels78]

@bachya thank you for the fix, I was going nuts without my notifications!

I have replaced the notify.py file with yours and it is back on track. will update officially when the fix is pushed!

[MartinHjelmare]

Looks good!