Opentherm gateway: Set unit of measurement for count sensors

[bajansen]

Proposed change

This PR sets the unit of measurement for the opentherm_gw 'starts' sensors to the empty string instead of None. This makes Home Assistant see the value of these sensors as a number and makes it 'graphable' instead of displaying it as a string.

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:

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
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• Tests have been added to verify that the new code works.

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• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
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• Untested files have been added to .coveragerc.

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[home-assistant]

Hey there @mvn23, mind taking a look at this pull request as it has been labeled with an integration (opentherm_gw) you are listed as a code owner for? Thanks!

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[home-assistant]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks 👍

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