I know a little of Carl Sagan as I worked at the time for the company that built the Digital cameras for the Voyager spacecraft and Carl was known as an agnostic but he also agreed one can neither prove nor disprove the existence of a higher being. As to the conjecture over how many stars/grains of sand this was essentially a metaphor to express enormity of scale first mooted to ill-educated people , and as far as we know so far by the Summerians and later incorporated into parts of the Bible, And the scientific community continues to do the same thing but caveating their answers with, could be, might be, estimates conclude etc etc. That is to say they can neither prove nor disprove definitively.