Rationally, you should say it's unknown in the absence of any evidence at all. You can't start assessing the validity of a hypothesis until you have at least some evidence to go by.
The examples you've given both have evidence to suggest the hypotheses were false, they are easily testable, and therefore verifiable.
If something is untestable by current methods, then we have no means of producing useful evidence either for or against the hypothesis. To take a stance on such a situation is therefore done based on what you believe. Many people believe, or if you like, put faith in, the idea of logical positivism. The idea that you should assume absence of evidence is evidence of absence, but there is nothing to say this belief is any more valid than any other. It's not testable as such, it's an assumption, a commonly used one, but still an assumption. Unfortunately proper application of it requires that evidence can be gathered should the hypothesis be true. If you aren't sure you can gather evidence, then using the idea is purely based on faith, not common sense. If you're using the wrong evidence gathering tool, and fail to gather evidence, have you proved the hypothesis false?