selftests/bpf: precision of a NULL global subprogram BTF_ID argument

Check that mark_chain_precision() is called for a NULL pointer passed
as an __arg_trusted __arg_nullable argument of a global subprogram.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260904-register-is-null-precise-fixes-v1-8-0f5a360ff15d@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>