Sort field is required for dumping

[CGamesPlay]

If the sort argument is not provided, the script fails:

Traceback (most recent call last):
  File "/usr/src/app/elastician/tools.py", line 372, in <module>
    cli()
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 1128, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 1053, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 1659, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 1395, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.7/site-packages/click/core.py", line 754, in invoke
    return __callback(*args, **kwargs)
  File "/usr/src/app/elastician/tools.py", line 132, in dump
    dump_func(index, es_source, timeout, size, query, sort)
  File "/usr/src/app/elastician/tools.py", line 211, in dump_func
    for d in tqdm(helpers.scan(es_source, index=index, query=q, sort={sort[0]: sort[1]},
TypeError: 'NoneType' object is not subscriptable