html/template: ignore untyped nil arguments to default escapers

html/template: ignore untyped nil arguments to default escapers

CL 95215 changed text/template so that untyped nil arguments were no
longer ignored, but were instead passed to functions as expected.
This had an unexpected effect on html/template, where all data is
implicitly passed to functions: originally untyped nil arguments were
not passed and were thus effectively ignored, but after CL 95215 they
were passed and were printed, typically as an escaped version of "<nil>".

This CL restores some of the behavior of html/template by ignoring
untyped nil arguments passed implicitly to escaper functions.

While eliminating one change to html/template relative to earlier
releases, this unfortunately introduces a different one: originally
values of interface type with the value nil were printed as an escaped
version of "<nil>". With this CL they are ignored as though they were
untyped nil values. My judgement is that this is a less common case.
We'll see.

This CL adds some tests of typed and untyped nil values to
html/template and text/template to capture the current behavior.

Updates #18716
Fixes #25875

Change-Id: I5912983ca32b31ece29e929e72d503b54d7b0cac
Reviewed-on: https://go-review.googlesource.com/121815
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>