When Morkrut Necropod attacks, its trigger doesn't resolve until after all attackers have been chosen. The same is true for when it blocks. This means that another creature, such as one enchanted with Invocation of Saint Traft, can also attack or block and then be sacrificed. Sacrificing a blocking creature this way doesn't cause the creature it blocked to become unblocked.

In the unusual situation that you control no other creatures or lands, the last ability won't do anything. Morkrut Necropod can attack or block with no penalty.