Fix missing column when parsed Fitbit sleep intraday data are empty

[jenniferfedor]

Recent changes to the mutation script for parsing raw Fitbit sleep intraday JSON data inadvertently resulted in the "local_date_time" column being excluded from the parsed data when the parsed data were empty. This caused the following error when the mutation script to add zero timestamps was subsequently applied to those data:

Error in py_call_impl(callable, dots$args, dots$keywords) : 
  KeyError: 'local_date_time'

Detailed traceback: 
  File "/rapids/src/data/streams/mutations/fitbit/add_zero_timestamp.py", line 5, in main
    if pd.api.types.is_datetime64_any_dtype(parsed_data['local_date_time']):

This PR fixes this issue.