Remove redundant empty-frame guard in FocusCsvSerializer

Polars write_csv already handles empty DataFrames correctly (outputs
header-only CSV), so the conditional branch was a no-op.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

litellm/integrations/focus/serializers/csv.py

@@ -16,7 +16,6 @@ class FocusCsvSerializer(FocusSerializer):
 
     def serialize(self, frame: pl.DataFrame) -> bytes:
         """Encode the provided frame as a CSV payload."""
-        target = frame if not frame.is_empty() else pl.DataFrame(schema=frame.schema)
         buffer = io.BytesIO()
-        target.write_csv(buffer)
+        frame.write_csv(buffer)
         return buffer.getvalue()