hexToInt raises exception on null string or "0x".

[aegisdino]

When hex string is null string or 0x hexToInt raises exception. So, hexToInt should be changed like the following.

///Takes the hexadecimal input and creates a [BigInt].
BigInt hexToInt(String hex) {
  if (hex == '' || hex == '0x') return BigInt.from(0);
  return BigInt.parse(strip0x(hex), radix: 16);
}

[TheGreatAxios]

@aegisdino Please create a PR