Remove null-deref in test

Dereferencing a nullptr is UB by spec, even if the method called is
non-virtual and no memory access to nullptr appears to actually occur.
For example:

error: 'this' pointer cannot be null in well-defined C++ code;
comparison may be assumed to always evaluate to true
[-Werror,-Wtautological-undefined-compare]

Bug: b/179840737
Change-Id: I6385e157e9a883cdd65f5ef9be89bcc3a29324fe
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/56720
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
Auto-Submit: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

src/tests/unittests/RefCountedTests.cpp

@@ -126,7 +126,7 @@ TEST(Ref, DefaultsToNull) {
     Ref<RCTest> test;
 
     EXPECT_EQ(test.Get(), nullptr);
-    EXPECT_EQ(test->GetThis(), nullptr);
+    // Can't check GetThis() returns nullptr, as it would be undefined behavior.
 }
 
 // Test Ref's copy constructor