Use the modern spelling for the nullability attributes in the test mock
headers. Currently, this was relying on the predefined macros from
clang to work. However, those are only available on Darwin targets.
This is needed to make the mock environments more portable.
This previously blew up if the Objective-C client passed NULL for the
error parameter, but started working after the pointer nullability
change. Why? John had /already written and committed/ code to handle
NULL assuming pointer nullability was explicit, and that code was
/correct as is/.
