XQuery: Check existence of paths in predicates

[ChristianGruen]

The following query…

db:open('factbook')/mondial/country/country

…will be rewritten to an empty sequence at compile time, because the country element has no country child elements.

The existence checks should be enhanced to also support steps with predicates…

db:open('factbook')/mondial/country/country[@population]

…and paths within predicates:

db:open('factbook')/mondial/country[country]

The optimizations could serve as preparatory step for #2069.