modest improvement to cropping (safer)

copienator/crop_blank_margins.py

@@ -110,7 +110,8 @@ def main() -> None:
               flush=True)
         (args.output/'report.json').write_text(json.dumps(rows, indent=2)+'\n')
     with (args.output/'report.csv').open('w') as stream:
-        writer = csv.DictWriter(stream, fieldnames=list(rows[0]))
+        fieldnames = list(dict.fromkeys(key for row in rows for key in row))
+        writer = csv.DictWriter(stream, fieldnames=fieldnames)
         writer.writeheader()
         writer.writerows(rows)
     cards = []