Rodn sonar recommendations cleaning

AoU-Access/src/main/java/ch/unige/aou/service/CleanFileAndThumbnailService.java

@@ -33,6 +33,7 @@ import java.util.Comparator;
 import java.util.List;
 import java.util.Objects;
 import java.util.Optional;
+import java.util.stream.Stream;
 
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -138,9 +139,8 @@ public class CleanFileAndThumbnailService {
       log.info("{} file(s) have been deleted", filesDeleted);
     }
 
-    try {
-      Files.walk(rootPath)
-              .sorted(Comparator.reverseOrder())
+    try (Stream<Path> filesStream = Files.walk(rootPath)) {
+      filesStream.sorted(Comparator.reverseOrder())
               .map(Path::toFile)
               .filter(File::isDirectory)
               .filter(f -> Objects.requireNonNull(f.listFiles()).length == 0)