log/slog: require entire Attr to be empty to elide

log/slog: require entire Attr to be empty to elide

Specify that Handlers should ignore zero-valued Attrs.

Implement that policy in the built-in handlers.

Fixes #59282.

Change-Id: I4430686b61f49bdac849ee300daaabfac9895849
Reviewed-on: https://go-review.googlesource.com/c/go/+/484095
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>