Add test for coverage

It's challenging to create a real unit test for this change without
constructing a real Lucene index, but at least this invokes the changed
code.

Signed-off-by: Michael Froh <[email protected]>

Author: msfroh

server/src/test/java/org/opensearch/index/OpenSearchTieredMergePolicyTests.java

@@ -32,9 +32,18 @@
 
 package org.opensearch.index;
 
+import org.apache.lucene.index.MergePolicy;
+import org.apache.lucene.index.SegmentCommitInfo;
+import org.apache.lucene.index.SegmentInfos;
 import org.apache.lucene.index.TieredMergePolicy;
+import org.apache.lucene.util.InfoStream;
+import org.apache.lucene.util.Version;
 import org.opensearch.test.OpenSearchTestCase;
 
+import java.io.IOException;
+import java.util.Collections;
+import java.util.Set;
+
 public class OpenSearchTieredMergePolicyTests extends OpenSearchTestCase {
 
     public void testDefaults() {
@@ -80,4 +89,32 @@ public void testSetDeletesPctAllowed() {
         policy.setDeletesPctAllowed(42);
         assertEquals(42, policy.regularMergePolicy.getDeletesPctAllowed(), 0);
     }
+
+    public void testFindDeleteMergesReturnsNullOnEmptySegmentInfos() throws IOException {
+        MergePolicy.MergeSpecification mergeSpecification = new OpenSearchTieredMergePolicy().findForcedDeletesMerges(
+            new SegmentInfos(Version.LATEST.major),
+            new MergePolicy.MergeContext() {
+                @Override
+                public int numDeletesToMerge(SegmentCommitInfo info) {
+                    return 0;
+                }
+
+                @Override
+                public int numDeletedDocs(SegmentCommitInfo info) {
+                    return 0;
+                }
+
+                @Override
+                public InfoStream getInfoStream() {
+                    return InfoStream.NO_OUTPUT;
+                }
+
+                @Override
+                public Set<SegmentCommitInfo> getMergingSegments() {
+                    return Collections.emptySet();
+                }
+            }
+        );
+        assertNull(mergeSpecification);
+    }
 }