Task without due date gives error

[isabellaalstrom]

Add a check for due date before parsing

Traceback (most recent call last):
  File "/config/custom_components/grocy/__init__.py", line 268, in async_update_tasks
    ] = await self.hass.async_add_executor_job(self.client.tasks)
  File "/usr/local/lib/python3.8/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/usr/local/lib/python3.8/site-packages/pygrocy/grocy.py", line 591, in tasks
    raw_tasks = self._api_client.get_tasks()
  File "/usr/local/lib/python3.8/site-packages/pygrocy/grocy_api_client.py", line 649, in get_tasks
    return [TaskResponse(data) for data in parsed_json]
  File "/usr/local/lib/python3.8/site-packages/pygrocy/grocy_api_client.py", line 649, in <listcomp>
    return [TaskResponse(data) for data in parsed_json]
  File "/usr/local/lib/python3.8/site-packages/pygrocy/grocy_api_client.py", line 458, in __init__
    self.due_date = parse_date(parsed_json.get("due_date"))
  File "/usr/local/lib/python3.8/site-packages/pygrocy/utils.py", line 10, in parse_date
    return iso8601.parse_date(input_value)
  File "/usr/local/lib/python3.8/site-packages/iso8601/iso8601.py", line 195, in parse_date
    raise ParseError("Unable to parse date string %r" % datestring)
iso8601.iso8601.ParseError: Unable to parse date string ''