Consequently ignore empty MQTT state payloads and set state to unknown on "None" payload

[jbouwh]

Breaking change

The behavior on processing empty and "None" state payloads for MQTT entities has been made more predictable for alarm_control_panel, climate, cover, device_tracker, lock, select, valve and water_heater items,

• A "None" (str) for a state payload generally means that the state becomes unknown.
• An empty payload is ignored.

Previously a "None" state could be considered invalid or was just ignored.

Proposed change

See also breaking change section. The handling of empty payloads and payloads rendering to "None" are handled more consequently now.

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: MQTT: Payload is not supported #117597
• Link to documentation pull request: Document mqtt state updates with empty or None values home-assistant.io#32864

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

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[home-assistant]

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

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[frenck]

@jbouwh A wild merge conflict appeared out of nowhere... Could you take a look? 🙈

../Frenck

[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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rebased and resolved merge conflicts

[jbouwh]

Thanks!