[FIX] Change check on type in converter NIFD (#1227)

clinica/iotools/converters/nifd_to_bids/nifd_utils.py

@@ -214,7 +214,7 @@ def dataset_to_bids(
         session_id=lambda df: df.visit.apply(
             lambda x: (
                 "ses-M" + x.strip("M").zfill(3)
-                if pd.api.types.is_string_dtype(x)
+                if isinstance(x, str)
                 else f"ses-M{(6 * (x - 1)):03d}"
             )
         ),