Address some GC comments

Follow-up from:
https://android-review.googlesource.com/#/c/159650/

Change-Id: Id14f29b4ce5b70b63fcb3e74f8503ae60a3ae444
diff --git a/runtime/gc/collector/mark_compact.cc b/runtime/gc/collector/mark_compact.cc
index 65e6b40..0623fd4 100644
--- a/runtime/gc/collector/mark_compact.cc
+++ b/runtime/gc/collector/mark_compact.cc
@@ -142,7 +142,7 @@
 
 inline mirror::Object* MarkCompact::MarkObject(mirror::Object* obj) {
   if (obj == nullptr) {
-    return obj;
+    return nullptr;
   }
   if (kUseBakerOrBrooksReadBarrier) {
     // Verify all the objects have the correct forward pointer installed.