net/http: ensure null body in Fetch response is not read

The Fetch API returns a null body if there is no response body,
on browsers that support streaming the response body. This
change ensures we check for both undefined and null bodies
before attempting to read the body.

Fixes #27196

Change-Id: I0da86b61284fe394418b4b431495e715a037f335
Reviewed-on: https://go-review.googlesource.com/131236


Reviewed-by: Richard Musiol <neelance@gmail.com>
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>