fix(NODE-3821): nullish check before using toBSON override function

[cplepage]

Description

In the serializer, when it comes to check for an override value (toBSON), changing the value check to compare with null and undefined instead of relying on the truthiness of value.

What is the motivation for this change?

Cannot extend BigInt prototype with.toBSON() since 0n === false

trying to make this work

BigInt.prototype.toBSON = function() {
    return Long.fromBits(Number(this.valueOf() & BigInt(0xffffffff)),
        Number(this.valueOf() >> BigInt(32)))
}

// src/parser/serializer.ts:769
// Is there an override value
if(value && value.toBSON){
    // this won't go through if value === 0n
    ...
}

...

// then trows error here since value hasn't been overidden
// src/parser/serializer.ts:779
else if (typeof value === 'bigint') {
    throw new BSONTypeError('Unsupported type BigInt, please use Decimal128');
}

[nbbeeken]

Hi @cplepage, thanks so much for contributing here! I think this is something we would certainly want to fix. I've gone ahead and created a JIRA ticket to track the work and I have some suggestions of improvements for ya. If you get the chance adding a test of your usecase to: test/node/to_bson_test.js would be a great help.

@dariakp @durran @baileympearson, Just wanna get y'all's attention here I'm suggesting we drop the throw case with toBSON not being of type 'function'. While very unlikely it seems wrong that our BSON library would not allow the document: { toBSON: 1 } to be serialized. That being said we could easily keep it, and revisit that at a later time.

[cplepage]

@nbbeeken good idea on the optional chaining, looks clean!
I added a test on the current particular case. Let me know your thoughts

[nbbeeken]

Hey @cplepage do you mind if I push up some changes of my own here? due to our support for older node versions the testing arrangement can be kinda strange, I can make sure we still get bigint coverage on later versions of node 🙂

[cplepage]

@nbbeeken please go ahead! Tried a few things for the tests, but CI never passed

[nbbeeken]

@cplepage Let me know if you see anything I missed, I'll ask the team for a final look now 🙂 thanks again!

[cplepage]

@nbbeeken Looks good! Thanks to you for the quick replies!!