Raise AttributeError on attempts to access unset oneof fields (#510)

tests/inputs/google_impl_behavior_equivalence/test_google_impl_behavior_equivalence.py

@@ -50,8 +50,10 @@ def test_bytes_are_the_same_for_oneof():
 
     # None of these fields were explicitly set BUT they should not actually be null
     # themselves
-    assert isinstance(message.foo, Foo)
-    assert isinstance(message2.foo, Foo)
+    assert not hasattr(message, "foo")
+    assert object.__getattribute__(message, "foo") == betterproto.PLACEHOLDER
+    assert not hasattr(message2, "foo")
+    assert object.__getattribute__(message2, "foo") == betterproto.PLACEHOLDER
 
     assert isinstance(message_reference.foo, ReferenceFoo)
     assert isinstance(message_reference2.foo, ReferenceFoo)