roscpp: fixed name resolution of "/"

This patches fixes a bug in roscpp's names module, that caused the string `"/"` to be resolved as empty string because the trailing slash was removed by `ros::names::clean("/")`. I guess this is not intended.

For example, the following code works to get a XmlRpcValue of all parameters:
```
XmlRpc::XmlRpcValue all_parameters;
assert(ros::param::get("", all_parameters));
```
while the following returns an error:
```
XmlRpc::XmlRpcValue all_parameters;
assert(ros::param::get("/", all_parameters));
```

Additionally, I am not sure if the result of the `ros::names::clean()` function was well-defined for empty strings.