Is there a simple way of testing if the generator has no items, like peek, hasNext, isEmpty, something along those lines?
The simple answer to your question: no, there is no simple way.  There are a whole lot of work-arounds.
There really shouldn't be a simple way, because of what generators are: a way to output a sequence of values without holding the sequence in memory.  So there's no backward traversal.
You could write a has_next function or maybe even slap it on to a generator as a method with a fancy decorator if you wanted to.
How do I know if a generator is empty from the start?