go/build/constraint: fix parsing of "// +build" (with no args)

"// +build" by itself was like "// +build !" - unsatisfiable.
Make it so again (right now it panics).

Fixes #44487.

Change-Id: Iacbc1398af6f988ef011f9f438e792eb62f8f434
Reviewed-on: https://go-review.googlesource.com/c/go/+/320829


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>