s3/utils: avoid erronous NO MEMORY detection

s3/utils: avoid erronous NO MEMORY detection

since 5cc3c1b5f6b0289f91c01b20989558badc28fd61 if we don't have
a realm specified either on cmdline or in conf file we try to
copy (talloc_strdup) a NULL variable which triggers a NO_MEMORY
error when we check the result of the copy

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15384

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Sun Jun  4 12:42:16 UTC 2023 on atb-devel-224

(cherry picked from commit 22ab42c1007775abca0b578744d4c18a85cda627)