text/template: differentiate nil from missing arg

reflect.Value is a struct and does not have a kind nor any flag for
untyped nils. As a result, it is tricky to differentiate when we're
missing a value, from when we have one but it is untyped nil.

We could start using *reflect.Value instead, to add one level of
indirection, using nil for missing values and new(reflect.Value) for
untyped nils. However, that is a fairly invasive change, and would also
mean unnecessary allocations.

Instead, use a special reflect.Value that depicts when a value is
missing. This is the case for the "final" reflect.Value in multiple
scenarios, such as the start of a pipeline. Give it a specific,
unexported type too, to make sure it cannot be mistaken for any other
valid value.

Finally, replace "final.IsValid()" with "final != missingVal", since
final.IsValid() will be false when final is an untyped nil.

Also add a few test cases, all different variants of the untyped nil
versus missing value scenario.

Fixes #18716.

Change-Id: Ia9257a84660ead5a7007fd1cced7782760b62d9d
Reviewed-on: https://go-review.googlesource.com/95215
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

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