Add Stripe.Emptyable type definition

[richardm-stripe]

r? @suz-stripe
cc @stripe/api-libraries

Defines Stripe.Emptyable<T> = null | '' | T type. Many params in the Stripe API has a "pass empty string to unset" idiom, and stripe-node converts nulls to empty strings over the wire. Presently, the type bindings encode all of these as T | null, but this is a bit confusing because some of the docstrings recommend empty strings.

I think adding an "Emptyable" wrapper like this will make the meaning of these type definitions a little bit more clear, and it also gives us a single place where we can document this behavior.

In VSCode, the present mouseover looks like

and the new one will be this:

It seems like VSCode omits mention of | null altogether, so the new mouseover is the only one that actually communicates the emptyable nature of the field .

[remi-stripe]

Approving because the code is correct. I don't know much about Typescript though and I'm not sure how this will be used and whether it should be named Empty instead of Emptyable but deferring to you here. Don't hesitate to ask a review from someone familiar with TS instead

[suz-stripe]

I think this looks ok and don't have any strong feelings about the term Emptyable. After rolling alternatives around in my head, this seems like the most descriptive and specific way of putting it.