Adds a test to check that block cipher modes accept a zero-length
input.

Fixes #17435.

Change-Id: Ie093c4cdff756b5c2dcb79342e167b3de5622389
Reviewed-on: https://go-review.googlesource.com/31070
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>