CL 12905 disallowed "Bob" <""@example.com> but inadvertently
also disallowed "" <bob@example.com>. Move the empty string
check to apply only in the addr-spec.

Fixes #14866.

Change-Id: Ia0b7a1a32810aa78157ae77bd0130b78154c460d
Reviewed-on: https://go-review.googlesource.com/32176Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>