go/printer: make empty lines break table alignment

In previous versions of Go including 1.10, an empty line would break the
alignment of elements within an expression list.

golang.org/cl/104755 changed the heuristic, with the side effect that
empty lines no longer broke the table alignment.

Reintroduce the behavior and add a regression test for it.

Fixes #26352.

Change-Id: I410bcff4cba25c7f8497d46bd7890a2c7ee11d46
Reviewed-on: https://go-review.googlesource.com/125260


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>