It has actually given me a good pause when I tried to put the sentence apart into grammatical pieces. Like at first it looked like there were 2 subjects in the sentence, which were это and ничья. But it's kind of nonsense if we talk grammar, right? So I think это must be an object actually. At least if you put it into another sentence that has a more obvious object like "Я вижу это", это stays exactly это. But if you put ничья in there, it'll change - "Я вижу ничью." So yeah, grammatically:

Это была ничья. Это - an object, была - predicate, ничья - subject. And since our subject is feminine its predicate has to be feminine as well, where there's such distinction of course and here, with a past tense verb and all, there's such a distinction.