Fix/union all empty list#5716

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Anipaleja wants to merge 3 commits intospotify:mainfrom

Anipaleja:fix/union-all-empty-list

Commits on Jul 1, 2025

Add clear error message for SCollection.unionAll with empty input
Anipaleja
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Point users to ScioContext.unionAll for empty-safe operations
Anipaleja
committed
Improve SCollection.unionAll error handling and documentation
Anipaleja
committed