Do not allow modbus config without entities

[janiversen]

Breaking change

The Modbus integration do not allow a configuration with the device only (no entities), this will create an issue, and the entry will not be loaded.

For the special cases (like some custom_components) where it is wanted, please add a dummy entity.

Proposed change

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)
• Deprecation (breaking change to happen in the future)
• 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:
  Modbus, configuration without entities home-assistant.io#31893

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

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[janiversen]

This starts getting irritating, it seems to be another integration failing all the times.