Do not throw exception when MeasurementDetails.mediaSize is empty

It is legitimate for StorageMeasurement.measureExactStorage to return without any user keys
added to the returned MeasurementDetails, when the following is true:

mVolume == null || !mVolume.isMountedReadable()

Bug: 22394495
Change-Id: Ie40bd468a487208d98dc56aae41fdad7098f41a5