Check if samples returned by get_sample_data are not None

Check if samples returned by the method get_sample_data
of a network statistics driver are not None.
As described in the docstring of the abstract method
if a driver is not able to return samples for a meter_name
it has to return None.

Closes-bug: #1304327
Change-Id: Id4b2000adc20078631cb8ce1ed7f84fce1a3165d
Merge "Check if samples returned by get_sample_data are not None"