Ticket #579 - Error messages encountered when using POSIX winsync
    
    Bug description: posix_group_fix_memberuid_callback registered by
    posixWinsyncCreateMemberOfTask calls an internal modify function
    even if there are no attributes to fix up.  The attempt fails as
    expected, but it logs cryptic errors in the error log:
      - slapi_modify_internal_set_pb: NULL parameter
      - allow_operation: component identity is NULL
    
    Fix description: This patch skips calling the fix up internal
    modify if there is no attributes to fix up.
    
    https://fedorahosted.org/389/ticket/579
    
    Reviewed by Rich (Thank you!!)
    (cherry picked from commit 74423d2eaef50949fd96ded8a2251834663e95e8)