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/runtime_callbacks_test.cc b/runtime/runtime_callbacks_test.cc
index f1e78b4..abe99e0 100644
--- a/runtime/runtime_callbacks_test.cc
+++ b/runtime/runtime_callbacks_test.cc
@@ -294,7 +294,7 @@
   const char* descriptor_y = "LY;";
   Handle<mirror::Class> h_Y(
       hs.NewHandle(class_linker_->FindClass(soa.Self(), descriptor_y, class_loader)));
-  ASSERT_TRUE(h_Y.Get() != nullptr);
+  ASSERT_TRUE(h_Y != nullptr);
 
   bool expect1 = Expect({ "PreDefine:LY; <art-gtest-XandY.jar>",
                           "PreDefine:LX; <art-gtest-XandY.jar>",