net/http/cgi: don't pass nil Body to the child handler

For server requests, the http.Request Body should not be nil.

Fixes #39190

Change-Id: I32de7b6c0f6ca55008fea9fd86089cda0a2dea62
Reviewed-on: https://go-review.googlesource.com/c/go/+/235137


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>