cmd/go/internal/work: omit modinfo line from cache key when empty

cmd/go/internal/work: omit modinfo line from cache key when empty

Cache keys are dumped in case of mismatch; an empty modinfo string
adds noise to that dump without a corresponding benefit.

For #52009.

Change-Id: I1b4cd85fa5ff920973552fc94977954f93622a32
Reviewed-on: https://go-review.googlesource.com/c/go/+/398059
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>