Don't raise XmlResponseInvalidError when using NodeRelations

[FisherTsai]

Function NodeRelations raises XmlResponseInvalidError when the target element isn't used in any relations. In this situation, OSM API do return an valid XML with an empty <osm> element (Check the definition here). I don't think raising XmlResponseInvalidError in this situation is a proper way to go. It might be better to return an empty list, just like the behavior in osmapi v1.0.0.

Consider to make a change for this? For this function, improving the error handling to deal with this or just rolling back to v1.0.0?

Notes that function NodeWays has the same issue.

[metaodi]

@FisherTsai thanks for reporting this issue. I'll look into it.