Resolves a namespace prefix to the corresponding namespace URI.
This method is used for validating the QName type, for example.
If the prefix is "" (empty string), it indicates
an unprefixed value. The callee
should resolve it as for an unprefixed
element, rather than for an unprefixed attribute.
If the prefix is "xml", then the callee must resolve
this prefix into "http://www.w3.org/XML/1998/namespace",
as defined in the XML Namespaces Recommendation.
Returns:
namespace URI of this prefix.
If the specified prefix is not declared,
the implementation must return null.