Adjust testHKmeans assertion after #132706 (#132786)

Author: iverase

muted-tests.yml

@@ -501,9 +501,6 @@ tests:
 - class: org.elasticsearch.repositories.SnapshotMetricsIT
   method: testSnapshotAPMMetrics
   issue: https://github.com/elastic/elasticsearch/issues/132731
-- class: org.elasticsearch.index.codec.vectors.cluster.HierarchicalKMeansTests
-  method: testHKmeans
-  issue: https://github.com/elastic/elasticsearch/issues/132771
 - class: org.elasticsearch.xpack.esql.qa.single_node.GenerativeForkIT
   method: test {csv-spec:lookup-join.MvJoinKeyFromRowExpanded}
   issue: https://github.com/elastic/elasticsearch/issues/132778

server/src/test/java/org/elasticsearch/index/codec/vectors/cluster/HierarchicalKMeansTests.java

@@ -46,8 +46,9 @@ public void testHKmeans() throws IOException {
             assertEquals(nVectors, soarAssignments.length);
             // verify no duplicates exist
             for (int i = 0; i < assignments.length; i++) {
-                assertTrue(soarAssignments[i] >= 0 && soarAssignments[i] < centroids.length);
-                assertNotEquals(assignments[i], soarAssignments[i]);
+                int soarAssignment = soarAssignments[i];
+                assertTrue(soarAssignment == -1 || (soarAssignment >= 0 && soarAssignment < centroids.length));
+                assertNotEquals(assignments[i], soarAssignment);
             }
         } else {
             assertEquals(0, soarAssignments.length);