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Many readers will guess, no doubt correctly, that Q{}local represents a name in no namespace, and that the NameTest Q{}* matches names in no namespace. However, the spec doesn't actually say so, and we shouldn't leave readers to guess. Using a zero-length string to represent the namespace-that-dare-not-speak-its-name is a common convention, but by no means universal.
I think I fixed this as part of bug 28896. The editor's draft says: "If the BracedURILiteral has no content (for example, Q{}invoice) then the namespace URI of the QName is absent."
See comment 1.