importer can't handle empty strings

re: #249 - The importer can't handle empty strings and raises an error. I have changed the statement to just raise a warning. Maybe it's even better to just adjust the regular expression in this statement to accept empty strings, not 100% sure about it. (I tried, but I fear my try doesn't only allow empty strings, but also invalid RDF strings...)
I have also written a test for this case.