selftests/bpf: Test allowing NULL buffer in dynptr slice

bpf_dynptr_slice(_rw) no longer requires a buffer for verification. If the
buffer is needed, but not present, the function will return NULL.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Link: https://lore.kernel.org/r/20230506013134.2492210-3-drosen@google.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>