STYLE: Prefer building an array with None to fill S0

Prefer building an array with `None` to fill `S0` if `S0` is `None`
instead of a list to increase consistency.

Author: jhlegarreta

src/nifreeze/model/dmri.py

@@ -141,7 +141,7 @@ def fit_predict(self, index: int, **kwargs):
         if n_models == 1:
             predicted, _ = _exec_predict(self._model, **(kwargs | {"gtab": gradient, "S0": S0}))
         else:
-            S0 = np.array_split(S0, n_models) if S0 is not None else [None] * n_models
+            S0 = np.array_split(S0, n_models) if S0 is not None else np.full(n_models, None)
 
             predicted = [None] * n_models