Fix Growatt mix device IndexError when chart data is empty

[frenck]

Proposed change

The Growatt Server coordinator crashes with IndexError: list index out of range when a mix device returns empty chart data. This happens because sorted_keys[-1] is accessed without checking if the list is empty.

The fix guards the timestamp calculation with an if sorted_keys: check, skipping the lastdataupdate field when no chart data is available.

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 "list index out of range" #157141
• This PR is related to issue:
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[home-assistant]

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

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[Copilot]

Pull request overview

Fixes a crash in the Growatt Server integration’s Mix device coordinator when chartData is empty by avoiding indexing into an empty list during last-update timestamp calculation.

Changes:

• Guard Mix lastdataupdate calculation so empty chartData no longer raises IndexError.
• Add a regression test that sets up a Mix device with empty chartData.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
homeassistant/components/growatt_server/coordinator.py	Adds an if sorted_keys: guard before reading sorted_keys[-1] to prevent IndexError on empty chart data.
tests/components/growatt_server/test_sensor.py	Adds a test covering Mix devices returning empty chartData during Classic API setup.