runtime: don't write unique string to trace if it's length zero

runtime: don't write unique string to trace if it's length zero

While we're here, document that ID 0 is implicitly assigned to an empty
set of data for both stacks and strings.

Change-Id: Ic52ff3a1132abc5a8f6f6c4e4357e31e6e7799fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/723061
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>