8346008: Fix recent NULL usage backsliding in Shenandoah

Reviewed-by: kbarrett, wkemper

Author: Sonia Zaldana Calles

src/hotspot/share/gc/shenandoah/shenandoahFreeSet.cpp

@@ -792,7 +792,7 @@ HeapWord* ShenandoahFreeSet::allocate_single(ShenandoahAllocRequest& req, bool&
   // Free set maintains mutator and collector partitions.  Normally, each allocates only from its partition,
   // except in special cases when the collector steals regions from the mutator partition.
 
-  // Overwrite with non-zero (non-NULL) values only if necessary for allocation bookkeeping.
+  // Overwrite with non-zero (non-null) values only if necessary for allocation bookkeeping.
 
   switch (req.type()) {
     case ShenandoahAllocRequest::_alloc_tlab:

src/hotspot/share/gc/shenandoah/shenandoahGenerationalEvacuationTask.cpp

@@ -202,7 +202,7 @@ void ShenandoahGenerationalEvacuationTask::promote_in_place(ShenandoahHeapRegion
   while (obj_addr < tams) {
     oop obj = cast_to_oop(obj_addr);
     if (marking_context->is_marked(obj)) {
-      assert(obj->klass() != nullptr, "klass should not be NULL");
+      assert(obj->klass() != nullptr, "klass should not be null");
       // This thread is responsible for registering all objects in this region.  No need for lock.
       scanner->register_object_without_lock(obj_addr);
       obj_addr += obj->size();