Steps

1. Specify zones for a sharded collection
2. Run reshardCollection with the zones argument specified, but empty.
3. The server will error out here.

It is legal to specify an empty argument for zones even if there already exist zones in the collection – doing so should be equivalent to saying "the collection resharded under the new key should not use zones."

The solution lies in either allowing an empty zones object at the level of the initial split code, or feeding the initial split code boost::none if the zones object is empty at the level of the resharding coordinator.