cmd/go/internal/vet: only set work.VetExplicit if the list of explicit flags is non-empty
Updates #35837
Fixes #37030
Change-Id: Ifd3435803622a8624bab55a0f3fbc8855025282f
Reviewed-on: https://go-review.googlesource.com/c/go/+/217897
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>