Returns None if the attribute is not on the element in null or
RDF namespace, otherwise returns the attribute value
Note that RDF M&S technically doesn't allow the qualified form,
But user/implementor consensu allows this under the
XML NS concept of global attrs
'...the expansion of the namespace-qualified tag name
(Generic Identifier) of E. This expansion is generated by
concatenating the namespace name given in the namespace
declaration with the LocalPart of the qualified name.'
...
'URI-References are resolved to resource identifiers by first
resolving the URI-reference to absolute form as specified by
[RFC 2396] using the base URI of the document in which the RDF
statements appear.'