Do not add empty Metadata tag to NativeEndpointReference

See http://community.jboss.org/message/528192#528192 . Basically that empty Metadata element, despite adding no real value info as it has not child/attributes (btw it's optional in the schema), appear to cause interoperability problems with Microsoft.

This might be caused by the NativeEndpointReference's metadata attribute field that's always initialized with an empty Elements object (differently from the W3CEndpointReference).