ENH: Use a boolean to tell whether to serialize the pandas df index

Use a boolean to tell whether to serialize the pandas dataframe index.

Fixes:
```
scripts/dwi_gp_estimation_error_analysis.py:220: error:
 No overload variant of "to_csv" of "NDFrame" matches argument types "Any", "str", "None", "str"  [call-overload]
scripts/dwi_gp_estimation_error_analysis.py:220: note: Possible overload variants:
scripts/dwi_gp_estimation_error_analysis.py:220: note:     def (...)
```

raised for example in:
https://github.com/nipreps/nifreeze/actions/runs/12437972140/job/34728973936#step:8:121

scripts/dwi_gp_estimation_error_analysis.py

@@ -217,7 +217,7 @@ def main() -> None:
             print(f"Finished {n}-fold cross-validation")
 
         scores_df = pd.DataFrame(scores)
-        scores_df.to_csv(args.output_scores, sep="\t", index=None, na_rep="n/a")
+        scores_df.to_csv(args.output_scores, sep="\t", index=False, na_rep="n/a")
 
         grouped = scores_df.groupby(["n_folds"])
         print(grouped[["rmse"]].mean())