text/template: compare reflect.Value instances differently

text/template: compare reflect.Value instances differently

To avoid false positives from the reflectvaluecompare checker #43993

Use v.IsValid() instead of

var zero reflect.Value
v != zero

Also avoid comparing directly with the singleton reflect.Value
representing a missing value. Detect the missing value by type instead.

Change-Id: I3a00d63cf61c077e7c7ae816474aa1f032be325b
Reviewed-on: https://go-review.googlesource.com/c/go/+/308769
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Ian Lance Taylor <iant@google.com>