https://go-review.googlesource.com/37812 prohibits empty encoded-text.
This CL accepts it again for backward compatibility.

Change-Id: I0e0840b501927f147160b999bb59d2d029ea314c
Reviewed-on: https://go-review.googlesource.com/40051
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>