Check for zero objects on a MPI rank/node case after LB runs

[lifflander]

What Needs to be Done?

After the load balancer runs, it's possible it generates a distributed where a rank/node has zero objects. That configuration might cause VT to hang (which it in the process of being fixed), but also may not be supported for some applications.

We are seeing a hang in EMPIRE after LB that might be that case, but it's impossible to know.

For now, let's check for that case after migrations and print out if that's the case. We could also reject migrations that cause that case to happen.