String conversion behavior update

[podkovyrin]

This update changed how parsing an empty string into BigInt works:
d2d29c1#diff-33404997836caa3c602713d04259952721517b81439ca84f263b778837c177bfR44

5.3.0:
When you used BigInt(""), it would create a BigInt with a zero value: BigInt(0).

5.4.0:
Now, using BigInt("") causes a crash due to a force unwrap

Ideally, the expected behavior should be to return either a zero value or nil.