Fix empty passthrough body replacements

[oskstr]

Maybe I'm just mistaken, but looked like it wasn't really possible to clear out the request or response body 🤔

The doc/types of these look like this:

    /**
     * A string or buffer that replaces the request body entirely.
     *
     * If this is specified, the upstream request will not wait for the original request
     * body, so this may make responses faster than they would be otherwise given large
     * request bodies or slow/streaming clients.
     */
    replaceBody?: string | Uint8Array | Buffer;

and pretty similar description for the response body variant.

Since both '' and undefined are falsy, it seems like the original body is kept when passing in an empty string.

If this is intended behavior, are there other ways to do it? 😊

[CLAassistant]

All committers have signed the CLA.

[pimterry]

Good find and good fix, thanks! I think I missed this because all my relevant empty cases use buffers, where an empty buffer is indeed truthy.

[pimterry]

I'll release this later today, I've got a couple of other changes to pull in as well.

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