spec: Clarify that namespaceURI can be empty
The previous wording in the spec didn’t make it clear that it isn’t required (but at least one of it and `localName` is required). This wording is based on the error checking present in `update-mime-database.c`, rather than some innate knowledge of how MIME information should work. Signed-off-by: Philip Withnall <pwithnall@endlessos.org>