Update gridfs.ts

[theBGuy]

PR Checklist

Verify that you did the following:

• [ x ] Opened an issue to track and discuss the problem you are trying to solve. (Someone else has this opened already)
• [ x ] Submitted the changes using a new branch in your fork of this repo.
• [ x ] Added test for the changes and checked that code coverage didn't diminished if possible. (No test added but checked code coverage)
• [ x ] Docs were updated using jsdoc style comments when necessary. (N/A)

Related issue

Issue: #560

Describe what you did

GridFSBucket doesn't pass the file into a callback anymore, instead it sets the value of gridFSFile when the finish event is triggered. I added a check so to see f was undefined (handles if someone is using old driver and f was passed though callback) if it was then get the file from writeStream.gridFSFile

Is this a breaking change?

No, fixes broken compatibility with current mongodb driver

• Yes
• [ x ] No