Translation:There is no water in the glass.
Chinese is highly contextual, and I think one could indicate multiple glasses from context (as the quantity is not explicitly stated). That said, you could also probably something like 这些杯子里没有水。(these glasses have no water in them) if you want to be explicit about which glasses and their plurality
Yes, 杯子 can mean cup, glass, mug, or tumbler. "Tea cup" is 茶杯 and "coffee cup" is 咖啡杯.
I think this should be accepted. For the Chinese to specifically be glass rather than any kind of cup I think it should be 玻璃杯. That said I think glass is probably an acceptable as a translation of 杯子 given the tendancy of the term "glass" to be used as a synonym for cup in some cases in English (i.e. It's sometimes used to refer to cups not made of glass).
It won't even accept "this glass has no water" I wonder where they get the "there" from in the answer. The usual problem of nothing ever being done to increase the amount of English replies that are valid.
I thought you were only suppose to be able to tell whether it was singular or plural from context
"In the glass there is no water." - no one really speaks like this, but it's also accepted