Fix the return of statistic with getting no sample

In mysql scenario, when getting statistics with no sample
(this can be presented with invalid value of sample filters), ceilometer
will raise an error about NoneType.
Like other API of ceilometer, it should return an empty list.

Change-Id: I07cbc3f03f7a5e6751e6851f14ed1cdc16ee4a4c
Closes-bug: #1292322
Merge "Fix the return of statistic with getting no sample"