`prevValue` ignored when no value provided

I want to change the value of a key only if the previous value is empty. If I attempt to do this using curl, the request succeeds when it really should fail:

```
% curl -L http://127.0.0.1:4001/v2/keys/foo -XPUT -d value=bar
{"action":"set","node":{"key":"/foo","value":"bar","modifiedIndex":19,"createdIndex":19}}

% curl -L http://127.0.0.1:4001/v2/keys/foo -XPUT -d value=foo -d prevValue=
{"action":"set","node":{"key":"/foo","value":"foo","modifiedIndex":20,"createdIndex":20},"prevNode":{"key":"/foo","value":"bar","modifiedIndex":19,"createdIndex":19}}
```