selftests: bpf: Fix test_bpf_nf for trusted args becoming default

selftests: bpf: Fix test_bpf_nf for trusted args becoming default

With trusted args now being the default, passing NULL to kfunc
parameters that are pointers causes verifier rejection rather than a
runtime error. The test_bpf_nf test was failing because it attempted to
pass NULL to bpf_xdp_ct_lookup() to verify runtime error handling.

Since the NULL check now happens at verification time, remove the
runtime test case that passed NULL to the bpf_tuple parameter and
instead add verification-time tests to ensure the verifier correctly
rejects programs that pass NULL to trusted arguments.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20260102180038.2708325-11-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>