selftests/bpf: do not pass NULL for non-nullable params in dummy_st_ops

selftests/bpf: do not pass NULL for non-nullable params in dummy_st_ops

dummy_st_ops.test_2 and dummy_st_ops.test_sleepable do not have their
'state' parameter marked as nullable. Update dummy_st_ops.c to avoid
passing NULL for such parameters, as the next patch would allow kernel
to enforce this restriction.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240424012821.595216-4-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>