MAINT cosmetic improvement in _non_trivial_radius test helper (scikit-learn#27486)

Author: ogrisel

sklearn/metrics/tests/test_pairwise_distances_reduction.py

@@ -228,8 +228,10 @@ def _non_trivial_radius(
     # on average. Yielding too many results would make the test slow (because
     # checking the results is expensive for large result sets), yielding 0 most
     # of the time would make the test useless.
-    if precomputed_dists is None and metric is None:
-        raise ValueError("Either metric or dists must be provided")
+    assert (
+        precomputed_dists is not None or metric is not None
+    ), "Either metric or precomputed_dists must be provided."
+
     if precomputed_dists is None:
         assert X is not None
         assert Y is not None