"What if there were no star in the sky?" also works even if you want to use the subjunctive.
Note that not all native speakers of English use or prefer this type of construction. Also, this use of "were" instead of "was" is sometimes now called irrealis and considered separate from the subjunctive (which is then used to refer only to constructions like "it's important that you be here early tomorrow").
It doesn't really. I'd immediately think "what about the rest of them?". "Not a star" works, but that's because it is made indefinite by the article. I wonder if the point here is that in English, dropping the article implies "the" in a way it doesn't in other languages.
At the end of the day there is only "the reader understands" and "the reader does not understand", with the former being the goal of the author. There is some benefit in using what is conventional to help achieve the desired outcome, but beyond that it matters not.
"What if there were no stars in the sky?"