BUG: Fill mask parameter for trivial model in model factory

Fill mask parameter for trivial model in model factory: extract the
`mask` named argument from the keyword arguments.

Fixes:
```
test/test_integration.py::test_proximity_estimator_trivial_model
  /home/runner/work/nifreeze/nifreeze/src/nifreeze/model/base.py:103:
 UserWarning: No mask provided; consider using a mask to avoid issues in model optimization.
    warn(mask_absence_warn_msg)
```

raised for example in:
https://github.com/nipreps/nifreeze/actions/runs/12871532342/job/35885011589?pr=54#step:11:1159

Author: jhlegarreta

src/nifreeze/model/base.py

@@ -50,7 +50,9 @@ def init(model="DTI", **kwargs):
 
         """
         if model.lower() in ("s0", "b0"):
-            return TrivialModel(predicted=kwargs.pop("S0"), gtab=kwargs.pop("gtab"))
+            return TrivialModel(
+                mask=kwargs.pop("mask"), predicted=kwargs.pop("S0"), gtab=kwargs.pop("gtab")
+            )
 
         if model.lower() in ("avgdwi", "averagedwi", "meandwi"):
             from nifreeze.model.dmri import AverageDWIModel