runtime: print irrelevant IDs as nil in schedtrace

runtime: print irrelevant IDs as nil in schedtrace

We currently print these as -1, but some are technically uint64. We can
be more explicit about their irrelevance by printing 'nil' rather than
-1.

Change-Id: I267fd8830564c75032bfe9176af59047f5a90202
Reviewed-on: https://go-review.googlesource.com/c/go/+/419441
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>