Landy: So in one statement sagan says a belief in a theist god is ludicrous then the next one goes on to say 'well, we can't prove it's not there'??
Not so. Sagan is addressing the idea of the god of the Abrahamic religions in the first quote. This is what he calls ludicrous.
In the second statement he is addressing the idea that there could be some other kind of "god," an impersonal being more akin to the detached, indifferent god of deism. It is impossible to say such a being does not exist.
How does one disprove the hypothetical existence of null space?
There is no contradiction as he is discussing two very different conceptions of "god" and the subsequent likelihood of their existence based on an examination of available evidence. In the first instance, there is no tangible evidence proving the existence of the desert god of the OT and plenty of evidence against that belief. In the second instance, there is--by definition--no definitive evidence either way, nor is there likely to be with the available tools we have at present. There is the possibility that may change in the future, but it is only a speculative possibility.