Return "null" cycles for deprecated callers.

Callers using the deprecated Pair<ZonedDateTime, ZonedDateTime>
interface don't always check hasNext() before calling next(), so
give them (null, null) results instead of throwing.

Bug: 78020489
Test: builds, boots
Change-Id: Ibaeac646fa4e12e3e748cd7c706524d51dbeaafe