cmd/compile/internal/pgo: allow and ignore profiles with no sample
Passing a profile with no sample is arguably not a user error.
Accept such a profile, and ignore it as it doesn't indicate any
optimizations. This also makes testing easier.
Change-Id: Iae49a4260e20757419643153f50d8d5d51478411
Reviewed-on: https://go-review.googlesource.com/c/go/+/448495
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Michael Pratt <mpratt@google.com>