Hide empty data usage cycles.

When building list of historical data usage cycles, omit cycles
that don't contain any data usage.  A bad NITZ or NTP fix may have
caused us to record some usage in 1970 or 2038, which would have
shown all the cycles in between.  Now we hide empty cycles to keep
things clean.

Bug: 18282691
Change-Id: Ib04c95d1411400ba9967d311b59402a26d39cfe3