sleep.get_total_sleep_time returns 0 instead of nan

[dado93]

The function pywearable.sleep.get_total_sleep_time returns 0 when all the columns of the sleepSummary are set to NaN. The function must return NaN in this case.

[Vaeliss]

Another way to solve this would be to consider (instead of the sum of the sleep stages columns) only the column durationInMs of the sleep summary, which already by definition doesn't consider awake periods. In this case the sleep stages durations would be nan, but total sleep time wouldn't.