Default behavior is for transforms to start processing input from an offset
equivalent to "the latest committed offset at deploy time". However, in
situations where no records are committed between the moment the transform
was deployed and the moment the transform starts, the timequery servicing
this logic returns an empty result, which is handled by returning offset::min,
which signals the transform to begin processing at the beginning of the topic.

This commit adjusts the logic in source::offset_at_timestamp to return nullopt
for an empty timequery result, and the handling of that nullopt to resolve
the transform start offset to latest.

Signed-off-by: Oren Leiman <[email protected]>
(cherry picked from commit 2401fa3)

Specifically in situations where no records are produced between
deploy time and start time.

Signed-off-by: Oren Leiman <[email protected]>
(cherry picked from commit 0f49b21)