Consuming empty FormData as text mistakenly returns the boundary value

This is covered by two separate WPT tests:
- request/request-consume-empty.any.js (Consume empty FormData request body as text)
- response/response-consume-empty.any.js (Consume empty FormData response body as text)

Instead of returning an empty string, we're instead returning the FormData object's boundary value, e.g. `--f24d463f4e75847bd55cc309d9f28b99--`