[release-branch.go1.15] net/mail: return error on empty address list

This restores the handling accidentally changed in CL 217377.

Fixes #40804
For #40803
For #36959

Change-Id: If77fbc0c2a1dde4799f760affdfb8dde9bcaf458
Reviewed-on: https://go-review.googlesource.com/c/go/+/248598


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Jeremy Fishman <jfishman@cloudflare.com>
(cherry picked from commit 3e636ab9)
Reviewed-on: https://go-review.googlesource.com/c/go/+/251167