Ranging over a nil slice is a no-op, so guarding it with a nil check is
not useful.

Found with honnef.co/go/tools/cmd/staticcheck.

Change-Id: I6ce56bb6805809ca29349257f10fd69c30611643
Reviewed-on: https://go-review.googlesource.com/54131
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>