[release-branch.go1.8] text/template: fix handling of empty blocks

[release-branch.go1.8] text/template: fix handling of empty blocks

This was a subtle bug introduced in the previous release's fix for
issue 16156.

The definition of empty template was broken, causing the answer
to depend on the order of templates in the map.

Fixes #16156 (for real).
Fixes #19294.
Fixes #19204.

Change-Id: I1cd915c94534cad3116d83bd158cbc28700510b9
Reviewed-on: https://go-review.googlesource.com/38420
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/39594
Reviewed-by: Rob Pike <r@golang.org>