Use NULL for the fifth argument in openzfs's function zfs_send_one(), used within be_export().
The fifth argument is a const char pointer used to represent the name of a redaction bookmark.
If no redaction bookmark is to be used, NULL is the appropriate argument. This patch
fixes a failing test for 'bectl_test:bectl_export_import'.
branch pulled from https://github.com/mattmacy/networking.git