Fix add_bid_document endpoint

Traceback (most recent call last):
  File "/procedure/prozorro_sale/procedure/errors.py", line 81, in request_errors_middleware
    return await handler(request)
  File "/usr/local/lib/python3.8/site-packages/prozorro_sale/auth/__init__.py", line 26, in token_middleware
    return await handler(request)
  File "/procedure/prozorro_sale/procedure/utils.py", line 165, in accept_language_middleware
    return await handler(request)
  File "/usr/local/lib/python3.8/site-packages/prozorro_sale/auth/__init__.py", line 36, in request_type_middleware
    return await handler(request)
  File "/usr/local/lib/python3.8/site-packages/prozorro_sale/auth/__init__.py", line 42, in request_context_middleware
    return await handler(request)
  File "/procedure/prozorro_sale/procedure/errors.py", line 116, in retry_on_concurrency_error_middleware
    return await handler(request)
  File "/procedure/prozorro_sale/procedure/errors.py", line 100, in handler
    return await func(request, *args, **kwargs)
  File "/procedure/prozorro_sale/procedure/utils.py", line 139, in request_unpack_params
    return await handler(request, **request.match_info)
  File "/procedure/prozorro_sale/procedure/utils.py", line 75, in handler
    return await func(request, *args, **kwargs)
  File "/procedure/prozorro_sale/procedure/api.py", line 430, in add_bid_document
    LOG.info(f'Document {doc_info["id"]} appended to bid {bid_id}')
TypeError: 'NoneType' object is not subscriptable```

[2020-03-19T16:07:15 +0000] | aiohttp.access | INFO - “159.224.64.180 POST /api/procedures/5e7398b39fd0d072849dc360/bid/b14703d35c324083bfde8253335b3a3f/documents?acc_token=%23censored%23 HTTP/1.1 500 212 - Swagger-Codegen/v3.0.11/python 0.012455”