ENH: Add None to b-vals iterators size parameter annotations

Add `None` to b-vals iterators `size` parameter annotations.

Fixes:
```
src/nifreeze/utils.py:30: error:
 Incompatible default for argument "size" (default has type "None", argument has type "int")  [assignment]
src/nifreeze/utils.py:30: note:
 PEP 484 prohibits implicit Optional. Accordingly, mypy has changed its default to no_implicit_optional=True
src/nifreeze/utils.py:30: note:
 Use https://github.com/hauntsaninja/no_implicit_optional to automatically upgrade your codebase
src/nifreeze/utils.py:59: error:
 Incompatible default for argument "size" (default has type "None", argument has type "int")  [assignment]
src/nifreeze/utils.py:59:
 note: PEP 484 prohibits implicit Optional. Accordingly, mypy has changed its default to no_implicit_optional=True
src/nifreeze/utils.py:59:
 note: Use https://github.com/hauntsaninja/no_implicit_optional to automatically upgrade your codebase
src/nifreeze/utils.py:108:
 error: Incompatible default for argument "size" (default has type "None", argument has type "int")  [assignment]
src/nifreeze/utils.py:108:
 note: PEP 484 prohibits implicit Optional. Accordingly, mypy has changed its default to no_implicit_optional=True
src/nifreeze/utils.py:108:
 note: Use https://github.com/hauntsaninja/no_implicit_optional to automatically upgrade your codebase
src/nifreeze/utils.py:135:
 error: Incompatible default for argument "size" (default has type "None", argument has type "int")  [assignment]
src/nifreeze/utils.py:135:
 note: PEP 484 prohibits implicit Optional. Accordingly, mypy has changed its default to no_implicit_optional=True
src/nifreeze/utils.py:135:
 note: Use https://github.com/hauntsaninja/no_implicit_optional to automatically upgrade your codebase
```

raised for example in:
https://github.com/nipreps/nifreeze/actions/runs/12437972140/job/34728973936#step:8:32

src/nifreeze/utils.py

@@ -27,15 +27,16 @@
 from typing import Iterator
 
 
-def linear_iterator(size: int = None, **kwargs) -> Iterator[int]:
+def linear_iterator(size: int | None = None, **kwargs) -> Iterator[int]:
     """
     Traverse the dataset volumes in ascending order.
 
     Parameters
     ----------
-    size : :obj:`int`
-        Number of volumes in the dataset
-        (for instance, the number of orientations in a DWI)
+    size : :obj:`int` or None, optional
+        Number of volumes in the dataset (for instance, the number of
+        orientations in a DWI). If `None`, the size is inferred from the `bvals`
+        argument in `kwargs`, if present.
 
     Returns
     -------
@@ -56,20 +57,21 @@ def linear_iterator(size: int = None, **kwargs) -> Iterator[int]:
     return range(size)
 
 
-def random_iterator(size: int = None, **kwargs) -> Iterator[int]:
+def random_iterator(size: int | None = None, **kwargs) -> Iterator[int]:
     """
     Traverse the dataset volumes randomly.
 
+    If the `seed` key is present in the keyword arguments, initializes the seed
+    of Python's `random` pseudo-random number generator library with the given
+    value. Specifically, if `False`, `None` is used as the seed; it `True`, a
+    default seed value is used.
+
     Parameters
     ----------
-    size : :obj:`int`
-        Number of volumes in the dataset
-        (for instance, the number of orientations in a DWI)
-    seed : :obj:`int` or :obj:`bool` or :obj:`bool` or ``None``
-        If :obj:`int` or :obj:`str` or ``None``, initializes the seed of Python's random generator
-        with the given value.
-        If ``False``, the random generator is passed ``None``.
-        If ``True``, a default seed value is set.
+    size : :obj:`int` or None, optional
+        Number of volumes in the dataset (for instance, the number of
+        orientations in a DWI). If `None`, the size is inferred from the `bvals`
+        argument in `kwargs`, if present.
 
     Returns
     -------
@@ -105,7 +107,7 @@ def random_iterator(size: int = None, **kwargs) -> Iterator[int]:
     return (x for x in index_order)
 
 
-def bvalue_iterator(size: int = None, **kwargs) -> Iterator[int]:
+def bvalue_iterator(size: int | None = None, **kwargs) -> Iterator[int]:
     """
     Traverse the volumes in a DWI dataset by growing b-value.
 
@@ -132,15 +134,16 @@ def bvalue_iterator(size: int = None, **kwargs) -> Iterator[int]:
     return (index[1] for index in indexed_bvals)
 
 
-def centralsym_iterator(size: int = None, **kwargs) -> Iterator[int]:
+def centralsym_iterator(size: int | None = None, **kwargs) -> Iterator[int]:
     """
     Traverse the dataset starting from the center and alternatingly progressing to the sides.
 
     Parameters
     ----------
-    size : :obj:`int`
-        Number of volumes in the dataset
-        (for instance, the number of orientations in a DWI)
+    size : :obj:`int` or None, optional
+        Number of volumes in the dataset (for instance, the number of
+        orientations in a DWI). If `None`, the size is inferred from the `bvals`
+        argument in `kwargs`, if present.
 
     Returns
     -------