findAndModify now returns T | null instead of T | undefined

[AsPulse]

According to the Official document of mongoDB, findAndModify returns the document or null.

I test this problem by below code:

const collection = /*...*/;
const result = await collection.findAndModify(
  {
    name: 'abc',
  },
  {
    update: {
      $set: {
        'someField': 12345,
      },
    },
    new: false,
  },
);

I've confirmed that the result is a pre-updated document if the document matches { name: 'abc' } found, if not, the result is certainly null.

Maybe, the return type should be Promise<T | null>.

Last but not least, I apologize if there are any mistakes in my English writing. I am just Japanese student learning English ;)

[lucsoft]

Looks great thanks very much for the PR!