Return null series with proper timestamp

A bit heavier, but will behave more like other storage engines.

For example, you have series:
host.something.foo
host.something.bar
host.something.baz

when you query for 'host.something.*' and, for some reasons, you don't receive 'host.something.bar' metric, sometimes you want to have "null" values in return, but in how currently graphite behaves, you won't get anything. This will fix ceres's library part of the problem - return list of nulls with correct retention period.