Revaluate if use of -1 value is intuitive

Found another case where a value of -1 is being used where it's possible that something like an optional type would be more intuitive: https://github.com/DiamondLightSource/TomoHub/blob/b98db1054e7e6a143b0e6a416bf4646f1192015e/frontend/src/components/crop/SelectionOperations.ts#L55-L57

It'd be worth quickly reviewing this and the context in which it's being used to see if a value of -1 is the best way to express the desired behaviour.