Refactor: Use WriteAndClearBuffer helper to prevent stale content

Addresses review feedback from HuaHuaY: Instead of clearing the buffer
in TranslateMinimalBatch for empty batches, use a WriteAndClearBuffer()
helper that writes and clears the buffer in all write paths.

This is cleaner because:
- Every write follows the same pattern (write -> clear)
- Easier to reason about for future write stages
- The invariant is explicit: buffer is always clean after flush

Author: rynewang

cpp/src/arrow/csv/writer.cc

@@ -541,7 +541,7 @@ class CSVWriterImpl : public ipc::RecordBatchWriter {
     for (auto maybe_slice : iterator) {
       ARROW_ASSIGN_OR_RAISE(std::shared_ptr<RecordBatch> slice, maybe_slice);
       RETURN_NOT_OK(TranslateMinimalBatch(*slice));
-      RETURN_NOT_OK(sink_->Write(data_buffer_));
+      RETURN_NOT_OK(WriteAndClearBuffer());
       stats_.num_record_batches++;
     }
     return Status::OK();
@@ -554,7 +554,7 @@ class CSVWriterImpl : public ipc::RecordBatchWriter {
     RETURN_NOT_OK(reader.ReadNext(&batch));
     while (batch != nullptr) {
       RETURN_NOT_OK(TranslateMinimalBatch(*batch));
-      RETURN_NOT_OK(sink_->Write(data_buffer_));
+      RETURN_NOT_OK(WriteAndClearBuffer());
       RETURN_NOT_OK(reader.ReadNext(&batch));
       stats_.num_record_batches++;
     }
@@ -590,6 +590,13 @@ class CSVWriterImpl : public ipc::RecordBatchWriter {
     return Status::OK();
   }
 
+  // GH-36889: Write buffer to sink and clear it to avoid stale content
+  // being written again if the next batch is empty.
+  Status WriteAndClearBuffer() {
+    RETURN_NOT_OK(sink_->Write(data_buffer_));
+    return data_buffer_->Resize(0, /*shrink_to_fit=*/false);
+  }
+
   int64_t CalculateHeaderSize(QuotingStyle quoting_style) const {
     int64_t header_length = 0;
     for (int col = 0; col < schema_->num_fields(); col++) {
@@ -654,13 +661,11 @@ class CSVWriterImpl : public ipc::RecordBatchWriter {
     next += options_.eol.size();
     DCHECK_EQ(reinterpret_cast<uint8_t*>(next),
               data_buffer_->data() + data_buffer_->size());
-    return sink_->Write(data_buffer_);
+    return WriteAndClearBuffer();
   }
 
   Status TranslateMinimalBatch(const RecordBatch& batch) {
     if (batch.num_rows() == 0) {
-      // GH-36889: Clear buffer to avoid writing stale content (e.g., header)
-      RETURN_NOT_OK(data_buffer_->Resize(0, /*shrink_to_fit=*/false));
       return Status::OK();
     }
     offsets_.resize(batch.num_rows());