Handle unparsable query strings by returning an empty tuple.

Fixes https://github.com/4teamwork/opengever.core/issues/588.

This is a very minimalistic approach to handle bad query strings. It would be nicer to present an error message to the user telling something about the concrete problem (`QueryParser` does raise `ParseError` with different error-messages). I think this should be done in a spearate step though ... maybe even in a different package, i.e. `ftw.tabbedview`.