Handle empty or missing state values for MQTT light entities using 'template' schema

[sumnerboy12]

Proposed change

Make handling of missing or empty state values more robust and ignore them rather than raising warning messages in the logs.

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 #141153
• This PR is related to issue:
• Link to documentation pull request:
• Link to developer documentation pull request:
• Link to frontend pull request:

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• The code change is tested and works locally.
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[home-assistant]

Hey there @emontnemery, @jbouwh, @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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Stale

[jbouwh]

Looks good, but can you add/update the tests

[jbouwh]

Please format your changes with ruff and add tests