s4/rpc_server: return NULL dns_name for NT4 trusts

s4/rpc_server: return NULL dns_name for NT4 trusts

That's what Windows returns for an NT4 trust:

  array: struct netr_DomainTrust
      netbios_name             : *
          netbios_name             : 'NT4TRUST'
      dns_name                 : NULL
      trust_flags              : 0x00000020 (32)
             0: NETR_TRUST_FLAG_IN_FOREST
             0: NETR_TRUST_FLAG_OUTBOUND
             0: NETR_TRUST_FLAG_TREEROOT
             0: NETR_TRUST_FLAG_PRIMARY
             0: NETR_TRUST_FLAG_NATIVE
             1: NETR_TRUST_FLAG_INBOUND
             0: NETR_TRUST_FLAG_MIT_KRB5
             0: NETR_TRUST_FLAG_AES
      parent_index             : 0x00000000 (0)
      trust_type               : LSA_TRUST_TYPE_DOWNLEVEL (1)
      trust_attributes         : 0x00000000 (0)
             0: LSA_TRUST_ATTRIBUTE_NON_TRANSITIVE
             0: LSA_TRUST_ATTRIBUTE_UPLEVEL_ONLY
             0: LSA_TRUST_ATTRIBUTE_QUARANTINED_DOMAIN
             0: LSA_TRUST_ATTRIBUTE_FOREST_TRANSITIVE
             0: LSA_TRUST_ATTRIBUTE_CROSS_ORGANIZATION
             0: LSA_TRUST_ATTRIBUTE_WITHIN_FOREST
             0: LSA_TRUST_ATTRIBUTE_TREAT_AS_EXTERNAL
             0: LSA_TRUST_ATTRIBUTE_USES_RC4_ENCRYPTION
             0: LSA_TRUST_ATTRIBUTE_CROSS_ORGANIZATION_NO_TGT_DELEGATION
             0: LSA_TRUST_ATTRIBUTE_PIM_TRUST
             0: LSA_TRUST_ATTRIBUTE_CROSS_ORGANIZATION_ENABLE_TGT_DELEGATION
      sid                      : *
          sid                      : S-1-5-21-4267984555-3675415144-1682400025
      guid                     : 00000000-0000-0000-0000-000000000000

Even though when creating the trust the DNS name must not be NULL and the
trustPartner and name attributes are set to the flatName in the trustedDomain
object:

  dn: CN=NT4TRUST,CN=System,DC=wdom2,DC=site
  objectClass: top
  objectClass: leaf
  objectClass: trustedDomain
  cn: NT4TRUST
  distinguishedName: CN=NT4TRUST,CN=System,DC=wdom2,DC=site
  instanceType: 4
  whenCreated: 20240118175040.0Z
  whenChanged: 20240118175040.0Z
  uSNCreated: 4939915
  uSNChanged: 4939916
  showInAdvancedViewOnly: TRUE
  name: NT4TRUST
  objectGUID: c2273b74-19ff-4f5a-b528-9e5ae21960dd
  securityIdentifier: S-1-5-21-4267984555-3675415144-1682400025
  trustDirection: 1
  trustPartner: NT4TRUST
  trustPosixOffset: 0
  trustType: 1
  trustAttributes: 0
  flatName: NT4TRUST
  objectCategory: CN=Trusted-Domain,CN=Schema,CN=Configuration,DC=wdom2,DC=site
  isCriticalSystemObject: TRUE
  dSCorePropagationData: 16010101000000.0Z

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15533

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>