last migration round only sends non-gas asset

we saw a churn get stuck due to all of the gas asset getting migrated but some of the non-gas asset was left behind. Since the network couldn't finish sending that non-gas asset, the churn was unable to be complete until someone donated ETH into the vault.

This PR changes the logic that the final round only transfer the non-gas assets (unless there are no non-gas assets). This would cause the network to have an "extra round" in its process, which seems acceptable to me.