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>
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