slog tries to represent a time.Time without allocations, which involves
storing its UnixNanos value. But UnixNanos is undefined for some valid
times. Provide a fallback representation for those times by storing them
in the `any` field of `Value`.
Fixes #65902.
Change-Id: I736c739a92f77d7b1122ea0831524acdd2c4703f
Reviewed-on: https://go-review.googlesource.com/c/go/+/585519
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Alan Donovan <adonovan@google.com>