Exception during CSV export

[jo-elimu]

Description

HTTP 500 Server Error during CSV export: http://eng.elimu.ai/analytics/students/4/letter-sound-learning-events.csv

https://github.com/elimu-ai/ml-datasets/actions/runs/15837372200/job/44643594431

• Add Discord notification (acba640)
• Add regression test
• Handle NullPointer exception

Why?

To restore organization-wide data pipeline.

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