go/printer: consider empty lines in table layout computation

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.

A prior fix (https://go-review.googlesource.com/c/go/+/125260, reverted)
introduced another regression (#26930) which this change doesn't produce.

Added test cases for both #26352 and #26930.

Fixes #26352.
Updates #26930.

Change-Id: I371f48e6f3620ebbab53f2128ec5e58bcd4a62f1
Reviewed-on: https://go-review.googlesource.com/129256


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