It's not really a null member, you need to pass in an array size of zero for the members parameter indicating that there are no members for the specific pool. That is not the same as passing in size 1 with a null value.
Pools are just buckets and can be empty. If you remove the last member in the pool, it will not delete the pool, it just won't have any members in it. A use case for this would be if you want to use an iRule to control the way connections flow to members, you can use the "node" command to target a connection to a specific node defined in the system.
Hope that helps...
-Joe
Nimbostratus