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 <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go