chacha20poly1305: add test for empty plaintext and additional data

The code (especially assembly implementations) should be able to
handle this scenario.

Change-Id: I68c6a5b8a099a23a87bfcb5f7246a2134f82bb9e
Reviewed-on: https://go-review.googlesource.com/112977
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>

chacha20poly1305/chacha20poly1305_vectors_test.go

@@ -7,6 +7,13 @@ package chacha20poly1305
 var chacha20Poly1305Tests = []struct {
 	plaintext, aad, key, nonce, out string
 }{
+	{
+		"",
+		"",
+		"808182838485868788898a8b8c8d8e8f909192939495969798999a9b9c9d9e9f",
+		"070000004041424344454647",
+		"a0784d7a4716f3feb4f64e7f4b39bf04",
+	},
 	{
 		"4c616469657320616e642047656e746c656d656e206f662074686520636c617373206f66202739393a204966204920636f756c64206f6666657220796f75206f6e6c79206f6e652074697020666f7220746865206675747572652c2073756e73637265656e20776f756c642062652069742e",
 		"50515253c0c1c2c3c4c5c6c7",