fix(node_evaluators): Handle the empty input list case

CHANGELOG.md

@@ -18,7 +18,7 @@ and the project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.
 - Add a top-level CLI for running end-to-end search ([#26](https://github.com/microsoft/syntheseus/pull/26)) ([@kmaziarz])
 - Release single-step evaluation framework and wrappers for several model types ([#14](https://github.com/microsoft/syntheseus/pull/14), [#15](https://github.com/microsoft/syntheseus/pull/15), [#20](https://github.com/microsoft/syntheseus/pull/20)) ([@kmaziarz])
 - Release checkpoints for all supported single-step model types ([#21](https://github.com/microsoft/syntheseus/pull/21)) ([@kmaziarz])
-- Implement node evaluators commonly used in MCTS and Retro* ([#23](https://github.com/microsoft/syntheseus/pull/23)) ([@kmaziarz])
+- Implement node evaluators commonly used in MCTS and Retro* ([#23](https://github.com/microsoft/syntheseus/pull/23), [#27](https://github.com/microsoft/syntheseus/pull/27)) ([@kmaziarz])
 - Add option to terminate search when the first solution is found ([#13](https://github.com/microsoft/syntheseus/pull/13)) ([@austint])
 - Add code to extract routes in order found instead of by minimum cost ([#9](https://github.com/microsoft/syntheseus/pull/9)) ([@austint])
 - Declare support for type checking ([#4](https://github.com/microsoft/syntheseus/pull/4)) ([@kmaziarz])

syntheseus/search/node_evaluation/base.py

@@ -138,6 +138,9 @@ def _get_probability(self, node, graph) -> float:
         return metadata["probability"]  # type: ignore
 
     def _evaluate_nodes(self, nodes, graph=None) -> Sequence[float]:
+        if not nodes:  # handle the case when there are no nodes to score
+            return []
+
         probs = np.asarray([self._get_probability(n, graph) for n in nodes])
         probs = np.clip(probs, a_min=self._clip_probability_min, a_max=self._clip_probability_max)