text/template: use "IsValid" instead of "!= zero" to compare zero Value

text/template: use "IsValid" instead of "!= zero" to compare zero Value

See CL 308769

Change-Id: I0caa0a84215b3d4b8b3dc6f041b6cd9cbe2c0908
GitHub-Last-Rev: e3aa5bde23fe26cbb6711a6ffbf4856a23fa6f54
GitHub-Pull-Request: golang/go#61657
Reviewed-on: https://go-review.googlesource.com/c/go/+/514195
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>