Allow `null` to reset time estimate in GraphQL mutation

The following discussion from !107892 (merged) should be addressed:

@leetickett-gitlab started a discussion: (+12 comments)

So omitting this argument or setting it to null will set the estimate to 0?

Do you think it would be better to make it required and more explicit/verbose?

Allowing to pass time_estimate as null in order to "remove" it seems logical, so I could easily adapt the code to do so, but I'd still allow user to set it to zero as it is a perfectly valid value, in fact it's the default value, but I'll defer to @gweaver just in case 🙂

And I definitely agree with @leetickett-gitlab, if we decide to go on a route, we should definitely document it so that others facing the same issue know what direction to take 🙂

Add support for using null to reset the time estimate - 0 is already supported.