ART: Add operator == and != with nullptr to Handle

Get it in line with ObjPtr and prettify our code.

Test: m
Change-Id: I1322e2a9bc7a85d7f2441034a19bf4d807b81a0e
diff --git a/runtime/class_table_test.cc b/runtime/class_table_test.cc
index f1248eb..18c2b82 100644
--- a/runtime/class_table_test.cc
+++ b/runtime/class_table_test.cc
@@ -80,7 +80,7 @@
   Handle<mirror::Class> h_Y(
       hs.NewHandle(class_linker_->FindClass(soa.Self(), descriptor_y, class_loader)));
   Handle<mirror::Object> obj_X = hs.NewHandle(h_X->AllocObject(soa.Self()));
-  ASSERT_TRUE(obj_X.Get() != nullptr);
+  ASSERT_TRUE(obj_X != nullptr);
   ClassTable table;
   EXPECT_EQ(table.NumZygoteClasses(class_loader.Get()), 0u);
   EXPECT_EQ(table.NumNonZygoteClasses(class_loader.Get()), 0u);