RDF-parser raises no error if an element has no SPDXID

[meretp]

When trying to parse a file that contains an element (file, package, snippet) without SPDXID no error is raised. Instead the element gets a random ID as it is a blank node (e.g. "N5c02cc1dd7c345b7abeaef214e1fa3e8"). The validation catches this as it is no valid SPDXID but I think this should raise an error when parsing.