text/template: fix type bug in eq

{{eq .x 0}} where .x is a nil interface{} should be false, not a type error.

Similarly, {{eq .x .x}} should succeed, not panic in reflect.

Fixes #45982.

Change-Id: I90aba82bb2f1a9e162bde1290c94f5028f56f412
Reviewed-on: https://go-review.googlesource.com/c/go/+/317470


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>