Nodereference path and url tokens return values for empty nodereferences

If you create an optional nodereference field for a node, and then attempt to retrieve the tokens for that node, all of the token values for the nodereference will be empty except for the path and url tokens, which will return "node" and "http://example.com/node" respectively. Those tokens should reflect the state of the referenced node. The attached patch fixes the token code to return an empty string for empty referenced nodes.