MAC/do: Fix a too stringent debug assertion for a target of 'uid=*'

MDF_HAS_PRIMARY_CLAUSE only concerns groups, not users, and is thus not
set in the latter case.

This change only has an effect on INVARIANTS builds.

PR: 287057
MFC after: 10 minutes
Sponsored by: The FreeBSD Foundation

(cherry picked from commit b5c9889e369a801ce7c1115f2535ddacbd69800d)