Make 5.7.2 consistent wrt context node

In 5.7.2, we say that the context node for a p:option is an empty
document node. For consistency with other changes, I think we should
say that it's an empty document node for XPath 1.0 implementations and
undefined for XPath 2.0 implementations.

For one thing, that'll catch this error:

  <p:option name="chapter" select="//chapter[1]"/>

which you'd think might be valid, but it isn't.

                                        Be seeing you,
                                          norm

-- 
Norman Walsh <ndw@nwalsh.com> | Advertising is not a means of
http://nwalsh.com/            | supporting media. Media is an excuse
                              | for presenting advertising.--Rusty
                              | Foster