cmd/go: report error for empty GOPROXY list

If GOPROXY is "", we set it to the default value,
"https://proxy.golang.org,direct". However, if GOPROXY is a non-empty
string that doesn't contain any URLs or keywords, we treat it as
either "off" or "noproxy", which can lead to some strange errors.

This change reports an error for this kind of GOPROXY value.

For #39180

Change-Id: If2e6e39d6f74c708e5ec8f90e9d4880e0e91894f
Reviewed-on: https://go-review.googlesource.com/c/go/+/234857


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>