[dev.fuzz] internal/fuzz: warn if fuzzing begins with empty corpus

Fixes golang/go#46219

Change-Id: I7b32707d490c046d15324a3d297ec8d9f45b6e36
Reviewed-on: https://go-review.googlesource.com/c/go/+/350269


Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>