test routing null matrix validation (#1220)

DataModel.add_cost_matrix routes through validate_matrix, which is expected to reject matrices containing NULL values before solve-time logic. This change adds a focused negative test to lock down that contract and prevent silent regressions during future validation.

### Summary

- Add test_dist_mat_null to routing validation coverage. 
- Cover the NULL input path for cost matrices in DataModel.add_cost_matrix.

Authors:
  - Daniel T. (https://github.com/aycsi)
  - Ramakrishna Prabhu (https://github.com/ramakrishnap-nv)
  - Trevor McKay (https://github.com/tmckayus)

Approvers:
  - Trevor McKay (https://github.com/tmckayus)

URL: #1220

Author: aycsi

python/cuopt/cuopt/tests/routing/test_warnings_exceptions.py

@@ -75,6 +75,21 @@ def test_dist_mat():
     )
 
 
+def test_dist_mat_null():
+    cost_matrix = cudf.DataFrame(
+        [
+            [0, 5.0, 5.0, 5.0],
+            [5.0, 0, 5.0, 5.0],
+            [5.0, 5.0, 0, 5.0],
+            [5.0, None, 5.0, 0],
+        ]
+    )
+    with pytest.raises(Exception) as exc_info:
+        dm = routing.DataModel(cost_matrix.shape[0], 3)
+        dm.add_cost_matrix(cost_matrix)
+    assert str(exc_info.value) == "cost matrix cannot have NULL values"
+
+
 def test_time_windows():
     cost_matrix = cudf.DataFrame(
         [