Handle invalid data returned by the server when asking for a batchId

JSONDecodeError: Expecting value: line 1 column 1 (char 0)
(8 additional frame(s) were not displayed)
...
  File "nuxeo/uploads.py", line 92, in post
  File "requests/models.py", line 900, in json
  File "__init__.py", line 346, in loads
  File "json/decoder.py", line 337, in decode
  File "json/decoder.py", line 355, in raw_decode