Irrelevant, because delete-null-pointer-checks happens even in absence of nonnull function attribute, see GP's godbolt link, and the documentation that omits any reference to that function attribute.
That is a side effect of passing the pointer as a function parameter marked nonnull. It implies that the pointer is nonnull and any NULL checks against it can be removed. Pass it to a normal function and you will not see the NULL check removed.
