Make `LogicValue` `toInt`, `toBigInt`, etc. return `null` when signal is invalid instead of an Exception

## Motivation
Dealing with null is easier with Dart's null safety than an exception.

## Desired solution
Functions that might not have a valid int/BigInt representation due to invalid signal values (x/z) should return `int?` and `BigInt?` instead of throwing an exception when it is invalid.

## Alternatives considered
Leave as it is with exceptions thrown when invalid.