There doesn't seem to be a direct translation of the empty() PHP function. However, you can easily add empty() to eZ Publish: http://ezpedia.org/wiki/en/ez/phpoperatorlist. Good luck ;)
Thanks for the reply. I wasn't actually only thinking about detecting empty values, but those, too. If I ever get across what's in my mind, I'll let you know ;)
Meanwhile, quite an interesting thing about PHPOperatorList as well as Wrap operator, even though at this point of my work I don't see a need or will to move too much programming to the presentation/template layer. Still, I will give it a closer look.