The aim of the PR is not to protect against an invalid input, but to handle correctly the null in JS.
JS interprets null as an object:

> typeof null
`object`

Therefore, the stringifyBigInts makes another recursion to call itself with null and the function breaks.
Hence, null needs to be handle differently.

I understand the stringifyBigInts as an utility to convert any object that contains bigInts into the same object, but the bigInts converted as strings, an object ready to be converted to a JSON file via JSON.stringify().
In the current approachm if that input object a null, the function breaks. While the JSON.stringify() works well with null