Fix handling `None` or empty value for numeric MQTT sensor

[jbouwh]

Breaking change

The behavior for receiving value on sensors that expect numeric* values has changed.

• From now on a None value will set such a sensor sensor to an unknown state.
• Empty values on such sensor '' are ignored an will not effect the state of the sensor. Integrations need to be corrected to send the correct values if an update is published and no valid update value is available for the sensor.

* Sensors expect numeric values if on of the following applies:

• device_class is set but is not a DATE, TIMESTAMP, ENUM or any custom device class.
• a state_class is set.
• unit_of_measurement is set.

Proposed change

MQTT sensors can accept string or numeric values. When device_class is set a numeric value is expected. Many configurations use value_templates when getting a sensor value from a published JSON. When a sensor has no value, and the device_class is set, this would give warnings in the log indicating an invalid non-numeric value was received.

This PR will change the behavior for MQTT sensors that have a device_class set and receive a value in two ways:

• ignore when empty string value '' is returned from the value_template for a numeric sensor.
• set a sensor to an unknown state when None as value us returned from the value template.

This is needed to allow maintainers get rid of warnings about invalid non-numeric in values in the logs asking people to open an issue.

example log for value rendering to an empty state:

Sensor sensor.power_dc has device class power, state class measurement and unit W thus indicating it has a numeric value; however, it has the non-numeric value: (<class 'str'>); Please update your configuration if your entity is manually configured, otherwise create a bug report at https://github.com/home-assistant/core/issues?q=is%3Aopen+is%3Aissue+label%3A%22integration%3A+mqtt%22

Type of change

• [ ] Dependency upgrade
• [x] 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)
• [x] 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 #87194
• This PR is related to issue:
• Link to documentation pull request: https://github.com/home-assistant/home-assistant.io/pull/26026

Checklist

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

If user exposed functionality or configuration variables are added/changed:

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

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[home-assistant[bot]]

Hey there @emontnemery, 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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[epenet]

Note: it is a breaking change, but should it be considered for 2023.2 or are we too close to release?

[emontnemery]

~~I think we should add this to 2023.2 since the alternative is a lot of warnings asking users to open issues.~~

[jbouwh]

@epenet We will not add this to 2023.2, that allows us up update the sensor base class first as you suggested. Will set this PR to draft for now.

[jbouwh]

~Would like to merge #87129 before this PR, and then update the docs PR to say that a numeric value is also expected if suggested_display_precision is set.~