embed: treat uninitialized FS as empty

As described in the FS documentation.

This prevents http.FS and other clients from panicking when the
go:embed directive is missing.

For #43682
Related #43698

Change-Id: Iecf26d229a099e55d24670c3119cd6c6d17ecc6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/283852


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>