Remove getattr for Landis+Gyr Heat Meter

[vpathuis]

Proposed change

As requested in #89937: removed getattr usage. Updated test cases to supply (now none optional) api-responses.

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 #
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[epenet]

LGTM 👍

[epenet]

Thanks @vpathuis