fix: handle DuckDB arrow() returning multiple types in duckdb_demo

[cning112]

Summary

• DuckDB's .arrow() can return RecordBatchReader, RecordBatch, or Table depending on the query
• Previously the code assumed only RecordBatchReader, causing type errors for queries returning single batches or tables
• Also skip empty batches to avoid downstream errors

Test plan

• Run duckdb_demo.py with various query types (aggregations, full table scans, single row)
• Verify no type errors from .arrow() return values

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