blackbox.ndrdump: fix test_ndrdump_fuzzed_NULL_struct_ntlmssp_CHALLENGE_MESSAGE test

blackbox.ndrdump: fix test_ndrdump_fuzzed_NULL_struct_ntlmssp_CHALLENGE_MESSAGE test

This actually reveals that ndr_push_string() for TargetName="" was
failing before because it resulted in 1 byte for a subcontext with
TargetLen=0.

This is fixed now and we no longer expect ndrdump to exit with 1.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14956

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Mon Jan 24 16:18:34 UTC 2022 on sn-devel-184

(cherry picked from commit 12464bd4c222d996aac6d6250b7945d63f20f4bc)

Autobuild-User(v4-16-test): Jule Anger <janger@samba.org>
Autobuild-Date(v4-16-test): Sun Jan 30 11:52:27 UTC 2022 on sn-devel-184