fix: prevent NPE in RemoteStorageHelper.cleanBuckets (#492)

The method getEventBasedHold and getTemporaryHold return Boolean objects
which are either true, false or null. In case of a null value, this
produced a NPE.

Author: Gerschtli

google-cloud-storage/src/main/java/com/google/cloud/storage/testing/RemoteStorageHelper.java

@@ -88,7 +88,8 @@ public void run() {
                                   Storage.BlobField.EVENT_BASED_HOLD,
                                   Storage.BlobField.TEMPORARY_HOLD))
                           .iterateAll()) {
-                    if (blob.getEventBasedHold() == true || blob.getTemporaryHold() == true) {
+                    if (Boolean.TRUE.equals(blob.getEventBasedHold())
+                        || Boolean.TRUE.equals(blob.getTemporaryHold())) {
                       storage.update(
                           blob.toBuilder().setTemporaryHold(false).setEventBasedHold(false).build(),
                           Storage.BlobTargetOption.userProject(