FIX: Disallow None values when initializaing PET data object

Disallow `None` values when initializaing PET data object: all
`frame_time`, `total_duration`, and `uptake` values are required to
build a `PET` data object. Thus, remove `None` from the type annotation
values.

Fixes:
```
typecheck: commands[0]> mypy .
src/nifreeze/estimator.py:269: error: Value of type "ndarray[Any, Any] | None" is not indexable  [index]
```

raised in:
https://github.com/nipreps/nifreeze/actions/runs/18052738286/job/51377557942?pr=203#step:8:34

Author: jhlegarreta

src/nifreeze/data/pet.py

@@ -45,15 +45,11 @@
 class PET(BaseDataset[np.ndarray | None]):
     """Data representation structure for PET data."""
 
-    midframe: np.ndarray | None = attrs.field(
-        default=None, repr=_data_repr, eq=attrs.cmp_using(eq=_cmp)
-    )
+    midframe: np.ndarray = attrs.field(default=None, repr=_data_repr, eq=attrs.cmp_using(eq=_cmp))
     """A (N,) numpy array specifying the midpoint timing of each sample or frame."""
-    total_duration: float | None = attrs.field(default=None, repr=True)
+    total_duration: float = attrs.field(default=None, repr=True)
     """A float representing the total duration of the dataset."""
-    uptake: np.ndarray | None = attrs.field(
-        default=None, repr=_data_repr, eq=attrs.cmp_using(eq=_cmp)
-    )
+    uptake: np.ndarray = attrs.field(default=None, repr=_data_repr, eq=attrs.cmp_using(eq=_cmp))
     """A (N,) numpy array specifying the uptake value of each sample or frame."""
 
     def _getextra(self, idx: int | slice | tuple | np.ndarray) -> tuple[np.ndarray | None]: