A complexType with mixed="true" and no sequence/choice/all (only attributes)
was compiled to the empty content type, which forbids text, so a mixed element
carrying only character data was rejected. It is now mixed content over an empty
particle: text is permitted, child elements are still disallowed. Non-mixed
empty types are unchanged.
XSTS: valid instances rejected 311 to 308. Baseline updated; regression tests
cover text accepted, an empty element, a child element rejected, and the
non-mixed control still rejecting text.
Refs #147, #129.
