avoid uninitlized value when urlto is called with 1 parameter and no url is configured

avoid uninitlized value when urlto is called with 1 parameter and no url is configured

This brings back the old behavior before urlto changes for this case.
It will generate a path like "/foo", which is not right, but is
the same as is generated by urlto($page, "", 1) -- which is what
the code that now uses 1-parameter urlto used to use.