LU-17892 lnet: Fix export with empty nets

LU-17892 lnet: Fix export with empty nets

lnetctl export --backup should not print an error when there haven't
been any NIs added.

Test-Parameters: trivial
Fixes: 8f8f6e2f36 ("LU-10003 lnet: use Netlink to support old and new NI APIs.")
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: Id9e916d2d70d5dc01442e24449cb787c5a6a7e1d
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/55279
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>