TypeError when completion json file doesn't exists

[sp1thas]

When I tried use Image object detection without initializing the completion files I noticed the following error:

Traceback (most recent call last):
  File "/home/username/label-studio/backend/utils.py", line 70, in exception_f
    return f(*args, **kwargs)
  File "/home/username/label-studio/backend/server.py", line 164, in api_completions
    completion_id = db.save_completion(task_id, completion)
  File "/home/username/label-studio/backend/db.py", line 199, in save_completion
    if 'completions' not in task:
TypeError: argument of type 'NoneType' is not iterable

the above error occurs then I hit "submit" because "$task_id".json is missing from the output folder.

I believe that we can bypass this unpleasant case by changing backend.db.get_completions function in that way so when the completion file is missing, an empty dictionary will be returned

[shevchenkonik]

@sp1thas Hey!

Please check the changes in the PR (#46).