FIX Output.Teardown bug

[choseenonee]

Fix: Safe Teardown Before First WriteRow

Previously, calling Teardown on a dataWriter before the first WriteRow caused a panic.
The issue occurred because the underlying writer (CSV or Parquet) was never initialized, yet Teardown attempted to flush it.

Example of failing flow

dataWriter := csv.NewWriter(
    ctx,
    w.model,
    w.config.CSVParams,
    w.columnsToDiscard,
    outPath,
    w.continueGeneration,
    w.writtenRowsChan,
)

dataWriter.Init()
dataWriter.Teardown() // panic here

Changes in this PR

• Fixed Teardown implementation for CSV and Parquet writers to handle cases when no rows were written.
• Added a unit test that reproduces the bug and verifies the fix.

[ReverseTM]

After we waited for writerWg and flushWg, no one else is using csvWriter. Do we need to take a lock? Please correct me if I've missed something.

[choseenonee]

Good point! You are absolutely right.