runtime: elide timer re-check if P has no timers

In golang.org/cl/264477, I missed this new block after rebasing past
golang.org/cl/232298. These fields must be zero if there are no timers.

Updates #28808
Updates #18237

Change-Id: I2d9e1cbf326497c833daa26b11aed9a1e12c2270
Reviewed-on: https://go-review.googlesource.com/c/go/+/266367


Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Pratt <mpratt@google.com>