s3: Don't set NextMarker when listing is not truncated (#7012)

Setting NextMarker when IsTruncated is not set seems to be confusing
AWS C++ SDK, this commit will avoid setting any string in NextMarker.

cmd/xl-v1-list-objects.go

@@ -130,15 +130,22 @@ func (xl xlObjects) listObjects(ctx context.Context, bucket, prefix, marker, del
 		xl.listPool.Set(params, walkResultCh, endWalkCh)
 	}
 
-	result := ListObjectsInfo{IsTruncated: !eof}
+	result := ListObjectsInfo{}
 	for _, objInfo := range objInfos {
-		result.NextMarker = objInfo.Name
 		if objInfo.IsDir && delimiter == slashSeparator {
 			result.Prefixes = append(result.Prefixes, objInfo.Name)
 			continue
 		}
 		result.Objects = append(result.Objects, objInfo)
 	}
+
+	if !eof {
+		result.IsTruncated = true
+		if len(objInfos) > 0 {
+			result.NextMarker = objInfos[len(objInfos)-1].Name
+		}
+	}
+
 	return result, nil
 }