Opening this per the request of Stephen M. over on the gremlin users forum.
If the type of a value in a GraphML file is specified as Double but the file contains an empty string in that element, a rather misleading exception is thrown that does not make it clear the XML file is the problem during the loading of the XML using the GraphML reader.
Stephen and I discussed a bit whether the exception text should just be improved or if the behavior should be changed (ie continue loading but issue a warning etc.)
As a side note, other GraphML implementations I tried, like Gephi, handle my XML file differently. Gephi loads the file and just does not assign any value to the properties that contain empty strings. That's not to say Gephi is right more observing this seems to be an area that is handled inconsistently across GraphML readers.