fixed `Task._update` method (`description` attribute)

[mtitov]

Race condition regarding having description attribute within task_dict

p.s., I couldn't spot from where specifically it comes, for most cases it is not the issue, but for some tasks it breaks the update process

Example of the error message

1702539334.001 : tmgr.0000            : 423200 : 140732555587952 : ERROR    : callback error
Traceback (most recent call last):
  File "/gpfs/alpine/csc449/world-shared/soma/summit/ve_workflow/lib/python3.8/site-packages/radical/utils/zmq/pubsub.py", line 304, in _listener
    cb(topic, msg)
  File "/gpfs/alpine/csc449/world-shared/soma/summit/ve_workflow/lib/python3.8/site-packages/radical/pilot/task_manager.py", line 358, in _state_sub_cb
    self._update_tasks(tasks)
  File "/gpfs/alpine/csc449/world-shared/soma/summit/ve_workflow/lib/python3.8/site-packages/radical/pilot/task_manager.py", line 402, in _update_tasks
    self._tasks[uid]._update(task_dict)
  File "/gpfs/alpine/csc449/world-shared/soma/summit/ve_workflow/lib/python3.8/site-packages/radical/pilot/task.py", line 181, in _update
    if task_dict['description'].get('metadata'):
KeyError: 'description'

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (7862aec) 44.28% compared to head (52664a5) 44.28%.

Files	Patch %	Lines
src/radical/pilot/task.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## devel #3105 +/- ## ======================================= Coverage 44.28% 44.28% ======================================= Files 96 96 Lines 10604 10604 ======================================= Hits 4696 4696 Misses 5908 5908 ```

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