wl status fails if there is no entry at all

[nils-braun]

When running wl status for the first time, it fails if no entry is created so far.

pip install dcs-worklog
> wl status # fine
Fatal: No log data available. Start a new log entry with 'wl commit start'.
> wl commit start -t 08:05
> wl status                                                                                                                                                                                           20-08-31 10:16
Traceback (most recent call last):
  File "/home/brn2rng/anaconda3/envs/worklog/lib/python3.8/site-packages/pandas/core/indexes/base.py", line 2889, in get_loc
    return self._engine.get_loc(casted_key)
  File "pandas/_libs/index.pyx", line 70, in pandas._libs.index.IndexEngine.get_loc
  File "pandas/_libs/index.pyx", line 97, in pandas._libs.index.IndexEngine.get_loc
  File "pandas/_libs/hashtable_class_helper.pxi", line 1675, in pandas._libs.hashtable.PyObjectHashTable.get_item
  File "pandas/_libs/hashtable_class_helper.pxi", line 1683, in pandas._libs.hashtable.PyObjectHashTable.get_item
KeyError: 'time'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/brn2rng/anaconda3/envs/worklog/bin/wl", line 8, in <module>
    sys.exit(run())
  File "/home/brn2rng/anaconda3/envs/worklog/lib/python3.8/site-packages/worklog/__init__.py", line 46, in run
    dispatch(log, parser, cli_args, cfg)
  File "/home/brn2rng/anaconda3/envs/worklog/lib/python3.8/site-packages/worklog/dispatcher.py", line 52, in dispatch
    log.status(hours_target, hours_max, query_date=query_date, fmt=fmt)
  File "/home/brn2rng/anaconda3/envs/worklog/lib/python3.8/site-packages/worklog/log.py", line 223, in status
    all_touched_tasks = get_all_task_ids_with_duration(self._log_df[sel_task_mask])
  File "/home/brn2rng/anaconda3/envs/worklog/lib/python3.8/site-packages/worklog/utils/tasks.py", line 42, in get_all_task_ids_with_duration
    s = df_h["time"]
  File "/home/brn2rng/anaconda3/envs/worklog/lib/python3.8/site-packages/pandas/core/frame.py", line 2902, in __getitem__
    indexer = self.columns.get_loc(key)
  File "/home/brn2rng/anaconda3/envs/worklog/lib/python3.8/site-packages/pandas/core/indexes/base.py", line 2891, in get_loc
    raise KeyError(key) from err
KeyError: 'time'