text/template recently added support for passing untyped nil as function
call arguments, as those would be mixed up with "missing argument"
values before. See CL 95215.

html/template now needs a small change to adapt to that new possibility.
In particular, when printing values as JS bytes, its code was written
under the assumption that the values would never be untyped nil - that
is, the reflect.Value would always be valid.

Short-circuit indirectToJSONMarshaler on an untyped nil, to avoid the
panic and fall back to the existing " null " output. Before this change
and on 1.10, printing a typed nil and an untyped nil resulted in:

	null ""

After this change, one will get:

	null null

Fixes #24717.

Change-Id: I03cd10ef64b96e837bacc9ccf4cf25624d80de1c
Reviewed-on: https://go-review.googlesource.com/109215
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rhys Hiltner <rhys@justin.tv>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>