[logdog] Fix Query library to only set timestamp constraints when supplied.
[logdog] Fix Query library to only set timestamp constraints when supplied.

Unfortunately the refactor to use the new proto timestamp libraries
inadvertently caused these fields to be populated as `Timestamp{0, 0}`
which is different from the previous `nil`.

R=vadimsh

Change-Id: I1110f691cf6458704670d51fa3c6c44aa3972992
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3077180
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Auto-Submit: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go