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

[mkorbel1]

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.

[mkorbel1]

It's worth considering which is better a little further. It might be good to look at other Dart conventions for toX types of functions.

[mkorbel1]

Instead of changing the behavior of the existing API, what if we add "try" versions like these: https://api.flutter.dev/flutter/dart-core/int/parse.html https://api.flutter.dev/flutter/dart-core/int/tryParse.html

For example, LogicValue.tryToInt()

[priya-t12]

I will work on this.